diff --git a/UPGRADE-5.3.md b/UPGRADE-5.3.md
index 32615caf15b6f..a69a34ce62ff8 100644
--- a/UPGRADE-5.3.md
+++ b/UPGRADE-5.3.md
@@ -9,6 +9,7 @@ Asset
 DoctrineBridge
 --------------
 
+ * Deprecate `UserLoaderInterface::loadUserByUsername()` in favor of `UserLoaderInterface::loadUserByIdentifier()
  * Remove `UuidV*Generator` classes
 
 DomCrawler
@@ -178,6 +179,11 @@ Security
    }
    ```
 
+ * Deprecate `UserInterface::getUsername()` in favor of `UserInterface::getUserIdentifier()`
+ * Deprecate `TokenInterface::getUsername()` in favor of `TokenInterface::getUserIdentifier()`
+ * Deprecate `UserProviderInterface::loadUserByUsername()` in favor of `UserProviderInterface::loadUserByIdentifier()`
+ * Deprecate `UsernameNotFoundException` in favor of `UserNotFoundException` and `getUsername()`/`setUsername()` in favor of `getUserIdentifier()`/`setUserIdentifier()`
+ * Deprecate `PersistentTokenInterface::getUsername()` in favor of `PersistentTokenInterface::getUserIdentifier()`
  * Deprecate calling `PasswordUpgraderInterface::upgradePassword()` with a `UserInterface` instance that does not implement `PasswordAuthenticatedUserInterface`
  * Deprecate calling methods `hashPassword()`, `isPasswordValid()` and `needsRehash()` on `UserPasswordHasherInterface` with a `UserInterface` instance that does not implement `PasswordAuthenticatedUserInterface`
  * Deprecate all classes in the `Core\Encoder\`  sub-namespace, use the `PasswordHasher` component instead
diff --git a/UPGRADE-6.0.md b/UPGRADE-6.0.md
index 997029ae61a6a..728475eed3710 100644
--- a/UPGRADE-6.0.md
+++ b/UPGRADE-6.0.md
@@ -6,6 +6,11 @@ Asset
 
  * Removed `RemoteJsonManifestVersionStrategy`, use `JsonManifestVersionStrategy` instead.
 
+DoctrineBridge
+--------------
+
+ * Remove `UserLoaderInterface::loadUserByUsername()` in favor of `UserLoaderInterface::loadUserByIdentifier()
+
 Config
 ------
 
@@ -262,6 +267,11 @@ Security
    }
    ```
 
+ * Remove `UserInterface::getUsername()` in favor of `UserInterface::getUserIdentifier()`
+ * Remove `TokenInterface::getUsername()` in favor of `TokenInterface::getUserIdentifier()`
+ * Remove `UserProviderInterface::loadUserByUsername()` in favor of `UserProviderInterface::loadUserByIdentifier()`
+ * Remove `UsernameNotFoundException` in favor of `UserNotFoundException` and `getUsername()`/`setUsername()` in favor of `getUserIdentifier()`/`setUserIdentifier()`
+ * Remove `PersistentTokenInterface::getUsername()` in favor of `PersistentTokenInterface::getUserIdentifier()`
  * Calling `PasswordUpgraderInterface::upgradePassword()` with a `UserInterface` instance that
    does not implement `PasswordAuthenticatedUserInterface` now throws a `\TypeError`.
  * Calling methods `hashPassword()`, `isPasswordValid()` and `needsRehash()` on `UserPasswordHasherInterface`
diff --git a/src/Symfony/Bridge/Doctrine/CHANGELOG.md b/src/Symfony/Bridge/Doctrine/CHANGELOG.md
index 7afda2ec2d167..7b627edbfa33b 100644
--- a/src/Symfony/Bridge/Doctrine/CHANGELOG.md
+++ b/src/Symfony/Bridge/Doctrine/CHANGELOG.md
@@ -4,6 +4,7 @@ CHANGELOG
 5.3
 ---
 
+ * Deprecate `UserLoaderInterface::loadUserByUsername()` in favor of `UserLoaderInterface::loadUserByIdentifier()
  * Deprecate `DoctrineTestHelper` and `TestRepositoryFactory`
  * [BC BREAK] Remove `UuidV*Generator` classes
  * Add `UuidGenerator`
diff --git a/src/Symfony/Bridge/Doctrine/Security/RememberMe/DoctrineTokenProvider.php b/src/Symfony/Bridge/Doctrine/Security/RememberMe/DoctrineTokenProvider.php
index 4116a6c9c6cb8..d2ac616db7439 100644
--- a/src/Symfony/Bridge/Doctrine/Security/RememberMe/DoctrineTokenProvider.php
+++ b/src/Symfony/Bridge/Doctrine/Security/RememberMe/DoctrineTokenProvider.php
@@ -119,7 +119,8 @@ public function createNewToken(PersistentTokenInterface $token)
             .' VALUES (:class, :username, :series, :value, :lastUsed)';
         $paramValues = [
             'class' => $token->getClass(),
-            'username' => $token->getUsername(),
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            'username' => method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername(),
             'series' => $token->getSeries(),
             'value' => $token->getTokenValue(),
             'lastUsed' => $token->getLastUsed(),
diff --git a/src/Symfony/Bridge/Doctrine/Security/User/EntityUserProvider.php b/src/Symfony/Bridge/Doctrine/Security/User/EntityUserProvider.php
index e0174e6bbb1b2..f965b8173f1ac 100644
--- a/src/Symfony/Bridge/Doctrine/Security/User/EntityUserProvider.php
+++ b/src/Symfony/Bridge/Doctrine/Security/User/EntityUserProvider.php
@@ -16,7 +16,7 @@
 use Doctrine\Persistence\ObjectManager;
 use Doctrine\Persistence\ObjectRepository;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -50,21 +50,35 @@ public function __construct(ManagerRegistry $registry, string $classOrAlias, str
      * {@inheritdoc}
      */
     public function loadUserByUsername(string $username)
+    {
+        trigger_deprecation('symfony/doctrine-bridge', '5.3', 'Method "%s()" is deprecated, use loadUserByIdentifier() instead.', __METHOD__);
+
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
     {
         $repository = $this->getRepository();
         if (null !== $this->property) {
-            $user = $repository->findOneBy([$this->property => $username]);
+            $user = $repository->findOneBy([$this->property => $identifier]);
         } else {
             if (!$repository instanceof UserLoaderInterface) {
                 throw new \InvalidArgumentException(sprintf('You must either make the "%s" entity Doctrine Repository ("%s") implement "Symfony\Bridge\Doctrine\Security\User\UserLoaderInterface" or set the "property" option in the corresponding entity provider configuration.', $this->classOrAlias, get_debug_type($repository)));
             }
 
-            $user = $repository->loadUserByUsername($username);
+            // @deprecated since 5.3, change to $repository->loadUserByIdentifier() in 6.0
+            if (method_exists($repository, 'loadUserByIdentifier')) {
+                $user = $repository->loadUserByIdentifier($identifier);
+            } else {
+                trigger_deprecation('symfony/doctrine-bridge', '5.3', 'Not implementing method "loadUserByIdentifier()" in user loader "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($repository));
+
+                $user = $repository->loadUserByUsername($identifier);
+            }
         }
 
         if (null === $user) {
-            $e = new UsernameNotFoundException(sprintf('User "%s" not found.', $username));
-            $e->setUsername($username);
+            $e = new UserNotFoundException(sprintf('User "%s" not found.', $identifier));
+            $e->setUserIdentifier($identifier);
 
             throw $e;
         }
@@ -96,8 +110,8 @@ public function refreshUser(UserInterface $user)
 
             $refreshedUser = $repository->find($id);
             if (null === $refreshedUser) {
-                $e = new UsernameNotFoundException('User with id '.json_encode($id).' not found.');
-                $e->setUsername(json_encode($id));
+                $e = new UserNotFoundException('User with id '.json_encode($id).' not found.');
+                $e->setUserIdentifier(json_encode($id));
 
                 throw $e;
             }
diff --git a/src/Symfony/Bridge/Doctrine/Security/User/UserLoaderInterface.php b/src/Symfony/Bridge/Doctrine/Security/User/UserLoaderInterface.php
index d996f71702291..b190eb249967a 100644
--- a/src/Symfony/Bridge/Doctrine/Security/User/UserLoaderInterface.php
+++ b/src/Symfony/Bridge/Doctrine/Security/User/UserLoaderInterface.php
@@ -22,16 +22,11 @@
  *
  * @see UserInterface
  *
+ * @method UserInterface|null loadUserByIdentifier(string $identifier) loads the user for the given user identifier (e.g. username or email).
+ *                                                                     This method must return null if the user is not found.
+ *
  * @author Michal Trojanowski <michal@kmt-studio.pl>
  */
 interface UserLoaderInterface
 {
-    /**
-     * Loads the user for the given username.
-     *
-     * This method must return null if the user is not found.
-     *
-     * @return UserInterface|null
-     */
-    public function loadUserByUsername(string $username);
 }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/BaseUser.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/BaseUser.php
index 416d5b20bf7b1..f03157637b256 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/BaseUser.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/BaseUser.php
@@ -37,4 +37,9 @@ public function getUsername(): string
     {
         return $this->username;
     }
+
+    public function getUserIdentifier(): string
+    {
+        return $this->username;
+    }
 }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/User.php b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/User.php
index f4ff7c2840ce7..0ffaf19883361 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Fixtures/User.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Fixtures/User.php
@@ -53,6 +53,11 @@ public function getUsername(): string
         return $this->name;
     }
 
+    public function getUserIdentifier(): string
+    {
+        return $this->name;
+    }
+
     public function eraseCredentials()
     {
     }
diff --git a/src/Symfony/Bridge/Doctrine/Tests/Security/User/EntityUserProviderTest.php b/src/Symfony/Bridge/Doctrine/Tests/Security/User/EntityUserProviderTest.php
index 216eee0e3f89c..20d1e487a23d2 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/Security/User/EntityUserProviderTest.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/Security/User/EntityUserProviderTest.php
@@ -21,7 +21,7 @@
 use Symfony\Bridge\Doctrine\Security\User\UserLoaderInterface;
 use Symfony\Bridge\Doctrine\Tests\DoctrineTestHelper;
 use Symfony\Bridge\Doctrine\Tests\Fixtures\User;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -60,7 +60,7 @@ public function testLoadUserByUsername()
 
         $provider = new EntityUserProvider($this->getManager($em), 'Symfony\Bridge\Doctrine\Tests\Fixtures\User', 'name');
 
-        $this->assertSame($user, $provider->loadUserByUsername('user1'));
+        $this->assertSame($user, $provider->loadUserByIdentifier('user1'));
     }
 
     public function testLoadUserByUsernameWithUserLoaderRepositoryAndWithoutProperty()
@@ -70,7 +70,7 @@ public function testLoadUserByUsernameWithUserLoaderRepositoryAndWithoutProperty
         $repository = $this->createMock(UserLoaderRepository::class);
         $repository
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with('user1')
             ->willReturn($user);
 
@@ -82,7 +82,7 @@ public function testLoadUserByUsernameWithUserLoaderRepositoryAndWithoutProperty
             ->willReturn($repository);
 
         $provider = new EntityUserProvider($this->getManager($em), 'Symfony\Bridge\Doctrine\Tests\Fixtures\User');
-        $this->assertSame($user, $provider->loadUserByUsername('user1'));
+        $this->assertSame($user, $provider->loadUserByIdentifier('user1'));
     }
 
     public function testLoadUserByUsernameWithNonUserLoaderRepositoryAndWithoutProperty()
@@ -98,7 +98,7 @@ public function testLoadUserByUsernameWithNonUserLoaderRepositoryAndWithoutPrope
         $em->flush();
 
         $provider = new EntityUserProvider($this->getManager($em), 'Symfony\Bridge\Doctrine\Tests\Fixtures\User');
-        $provider->loadUserByUsername('user1');
+        $provider->loadUserByIdentifier('user1');
     }
 
     public function testRefreshUserRequiresId()
@@ -126,7 +126,7 @@ public function testRefreshInvalidUser()
         $provider = new EntityUserProvider($this->getManager($em), 'Symfony\Bridge\Doctrine\Tests\Fixtures\User', 'name');
 
         $user2 = new User(1, 2, 'user2');
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
         $this->expectExceptionMessage('User with id {"id1":1,"id2":2} not found');
 
         $provider->refreshUser($user2);
@@ -153,7 +153,7 @@ public function testLoadUserByUserNameShouldLoadUserWhenProperInterfaceProvided(
     {
         $repository = $this->createMock(UserLoaderRepository::class);
         $repository->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with('name')
             ->willReturn(
                 $this->createMock(UserInterface::class)
@@ -164,7 +164,7 @@ public function testLoadUserByUserNameShouldLoadUserWhenProperInterfaceProvided(
             'Symfony\Bridge\Doctrine\Tests\Fixtures\User'
         );
 
-        $provider->loadUserByUsername('name');
+        $provider->loadUserByIdentifier('name');
     }
 
     public function testLoadUserByUserNameShouldDeclineInvalidInterface()
@@ -177,7 +177,7 @@ public function testLoadUserByUserNameShouldDeclineInvalidInterface()
             'Symfony\Bridge\Doctrine\Tests\Fixtures\User'
         );
 
-        $provider->loadUserByUsername('name');
+        $provider->loadUserByIdentifier('name');
     }
 
     public function testPasswordUpgrades()
@@ -231,6 +231,7 @@ private function createSchema($em)
 
 abstract class UserLoaderRepository implements ObjectRepository, UserLoaderInterface
 {
+    abstract public function loadUserByIdentifier(string $identifier): ?UserInterface;
 }
 
 abstract class PasswordUpgraderRepository implements ObjectRepository, PasswordUpgraderInterface
diff --git a/src/Symfony/Bridge/Monolog/Processor/AbstractTokenProcessor.php b/src/Symfony/Bridge/Monolog/Processor/AbstractTokenProcessor.php
index ed37c94b81c00..15919978857c3 100644
--- a/src/Symfony/Bridge/Monolog/Processor/AbstractTokenProcessor.php
+++ b/src/Symfony/Bridge/Monolog/Processor/AbstractTokenProcessor.php
@@ -42,10 +42,16 @@ public function __invoke(array $record): array
 
         if (null !== $token = $this->getToken()) {
             $record['extra'][$this->getKey()] = [
-                'username' => $token->getUsername(),
                 'authenticated' => $token->isAuthenticated(),
                 'roles' => $token->getRoleNames(),
             ];
+
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            if (method_exists($token, 'getUserIdentifier')) {
+                $record['extra'][$this->getKey()]['username'] = $record['extra'][$this->getKey()]['user_identifier'] = $token->getUserIdentifier();
+            } else {
+                $record['extra'][$this->getKey()]['username'] = $token->getUsername();
+            }
         }
 
         return $record;
diff --git a/src/Symfony/Bridge/Monolog/Tests/Processor/SwitchUserTokenProcessorTest.php b/src/Symfony/Bridge/Monolog/Tests/Processor/SwitchUserTokenProcessorTest.php
index 7107993b9c849..7d9aaede008c4 100644
--- a/src/Symfony/Bridge/Monolog/Tests/Processor/SwitchUserTokenProcessorTest.php
+++ b/src/Symfony/Bridge/Monolog/Tests/Processor/SwitchUserTokenProcessorTest.php
@@ -37,11 +37,15 @@ public function testProcessor()
 
         $expected = [
             'impersonator_token' => [
-                'username' => 'original_user',
                 'authenticated' => true,
                 'roles' => ['ROLE_SUPER_ADMIN'],
+                'username' => 'original_user',
             ],
         ];
-        $this->assertSame($expected, $record['extra']);
+        if (method_exists($originalToken, 'getUserIdentifier')) {
+            $expected['impersonator_token']['user_identifier'] = 'original_user';
+        }
+
+        $this->assertEquals($expected, $record['extra']);
     }
 }
diff --git a/src/Symfony/Bridge/Monolog/Tests/Processor/TokenProcessorTest.php b/src/Symfony/Bridge/Monolog/Tests/Processor/TokenProcessorTest.php
index dcaf0f647e301..b9fa51658e0d4 100644
--- a/src/Symfony/Bridge/Monolog/Tests/Processor/TokenProcessorTest.php
+++ b/src/Symfony/Bridge/Monolog/Tests/Processor/TokenProcessorTest.php
@@ -23,8 +23,12 @@
  */
 class TokenProcessorTest extends TestCase
 {
-    public function testProcessor()
+    public function testLegacyProcessor()
     {
+        if (method_exists(UsernamePasswordToken::class, 'getUserIdentifier')) {
+            $this->markTestSkipped('This test requires symfony/security-core <5.3');
+        }
+
         $token = new UsernamePasswordToken('user', 'password', 'provider', ['ROLE_USER']);
         $tokenStorage = $this->createMock(TokenStorageInterface::class);
         $tokenStorage->method('getToken')->willReturn($token);
@@ -38,4 +42,24 @@ public function testProcessor()
         $this->assertEquals($token->isAuthenticated(), $record['extra']['token']['authenticated']);
         $this->assertEquals(['ROLE_USER'], $record['extra']['token']['roles']);
     }
+
+    public function testProcessor()
+    {
+        if (!method_exists(UsernamePasswordToken::class, 'getUserIdentifier')) {
+            $this->markTestSkipped('This test requires symfony/security-core 5.3+');
+        }
+
+        $token = new UsernamePasswordToken('user', 'password', 'provider', ['ROLE_USER']);
+        $tokenStorage = $this->createMock(TokenStorageInterface::class);
+        $tokenStorage->method('getToken')->willReturn($token);
+
+        $processor = new TokenProcessor($tokenStorage);
+        $record = ['extra' => []];
+        $record = $processor($record);
+
+        $this->assertArrayHasKey('token', $record['extra']);
+        $this->assertEquals($token->getUserIdentifier(), $record['extra']['token']['user_identifier']);
+        $this->assertEquals($token->isAuthenticated(), $record['extra']['token']['authenticated']);
+        $this->assertEquals(['ROLE_USER'], $record['extra']['token']['roles']);
+    }
 }
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/Bundle/TestBundle/Controller/SecurityController.php b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/Bundle/TestBundle/Controller/SecurityController.php
index 6bf27e1ca2d9d..540a5b088a6b1 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/Bundle/TestBundle/Controller/SecurityController.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/Functional/Bundle/TestBundle/Controller/SecurityController.php
@@ -21,6 +21,6 @@ class SecurityController implements ContainerAwareInterface
 
     public function profileAction()
     {
-        return new Response('Welcome '.$this->container->get('security.token_storage')->getToken()->getUsername().'!');
+        return new Response('Welcome '.$this->container->get('security.token_storage')->getToken()->getUserIdentifier().'!');
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/DataCollector/SecurityDataCollector.php b/src/Symfony/Bundle/SecurityBundle/DataCollector/SecurityDataCollector.php
index 9bd7c005757bb..e346d34800413 100644
--- a/src/Symfony/Bundle/SecurityBundle/DataCollector/SecurityDataCollector.php
+++ b/src/Symfony/Bundle/SecurityBundle/DataCollector/SecurityDataCollector.php
@@ -97,7 +97,9 @@ public function collect(Request $request, Response $response, \Throwable $except
 
             $impersonatorUser = null;
             if ($token instanceof SwitchUserToken) {
-                $impersonatorUser = $token->getOriginalToken()->getUsername();
+                $originalToken = $token->getOriginalToken();
+                // @deprecated since 5.3, change to $originalToken->getUserIdentifier() in 6.0
+                $impersonatorUser = method_exists($originalToken, 'getUserIdentifier') ? $originalToken->getUserIdentifier() : $originalToken->getUsername();
             }
 
             if (null !== $this->roleHierarchy) {
@@ -126,7 +128,8 @@ public function collect(Request $request, Response $response, \Throwable $except
                 'token' => $token,
                 'token_class' => $this->hasVarDumper ? new ClassStub(\get_class($token)) : \get_class($token),
                 'logout_url' => $logoutUrl,
-                'user' => $token->getUsername(),
+                // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+                'user' => method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername(),
                 'roles' => $assignedRoles,
                 'inherited_roles' => array_unique($inheritedRoles),
                 'supports_role_hierarchy' => null !== $this->roleHierarchy,
diff --git a/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/UserProvider/InMemoryFactory.php b/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/UserProvider/InMemoryFactory.php
index a19a9eb163b3f..ceb04e340c8ea 100644
--- a/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/UserProvider/InMemoryFactory.php
+++ b/src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/UserProvider/InMemoryFactory.php
@@ -48,8 +48,30 @@ public function addConfiguration(NodeDefinition $node)
             ->fixXmlConfig('user')
             ->children()
                 ->arrayNode('users')
-                    ->useAttributeAsKey('name')
+                    ->useAttributeAsKey('identifier')
                     ->normalizeKeys(false)
+                    ->beforeNormalization()
+                        ->always()
+                        ->then(function ($v) {
+                            $deprecation = false;
+                            foreach ($v as $i => $child) {
+                                if (!isset($child['name'])) {
+                                    continue;
+                                }
+
+                                $deprecation = true;
+
+                                $v[$i]['identifier'] = $child['name'];
+                                unset($v[$i]['name']);
+                            }
+
+                            if ($deprecation) {
+                                trigger_deprecation('symfony/security-bundle', '5.3', 'The "in_memory.user.name" option is deprecated, use "identifier" instead.');
+                            }
+
+                            return $v;
+                        })
+                    ->end()
                     ->prototype('array')
                         ->children()
                             ->scalarNode('password')->defaultNull()->end()
diff --git a/src/Symfony/Bundle/SecurityBundle/Resources/config/schema/security-1.0.xsd b/src/Symfony/Bundle/SecurityBundle/Resources/config/schema/security-1.0.xsd
index 2196017ae99a9..f7b9790f4a810 100644
--- a/src/Symfony/Bundle/SecurityBundle/Resources/config/schema/security-1.0.xsd
+++ b/src/Symfony/Bundle/SecurityBundle/Resources/config/schema/security-1.0.xsd
@@ -136,7 +136,8 @@
     </xsd:complexType>
 
     <xsd:complexType name="user">
-        <xsd:attribute name="name" type="xsd:string" use="required" />
+        <xsd:attribute name="identifier" type="xsd:string" />
+        <xsd:attribute name="name" type="xsd:string" />
         <xsd:attribute name="password" type="xsd:string" />
         <xsd:attribute name="roles" type="xsd:string" />
     </xsd:complexType>
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/CompleteConfigurationTest.php b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/CompleteConfigurationTest.php
index a3c0cd6ffb0cf..f192ee614cbb6 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/CompleteConfigurationTest.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/CompleteConfigurationTest.php
@@ -297,7 +297,7 @@ public function testAccess()
             } elseif (3 === $i) {
                 $this->assertEquals('IS_AUTHENTICATED_ANONYMOUSLY', $attributes[0]);
                 $expression = $container->getDefinition((string) $attributes[1])->getArgument(0);
-                $this->assertEquals("token.getUsername() matches '/^admin/'", $expression);
+                $this->assertEquals("token.getUserIdentifier() matches '/^admin/'", $expression);
             }
         }
     }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/container1.php b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/container1.php
index f551131f00639..6118929a36f69 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/container1.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/container1.php
@@ -97,7 +97,7 @@
     'access_control' => [
         ['path' => '/blog/524', 'role' => 'ROLE_USER', 'requires_channel' => 'https', 'methods' => ['get', 'POST'], 'port' => 8000],
         ['path' => '/blog/.*', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY'],
-        ['path' => '/blog/524', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY', 'allow_if' => "token.getUsername() matches '/^admin/'"],
+        ['path' => '/blog/524', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY', 'allow_if' => "token.getUserIdentifier() matches '/^admin/'"],
     ],
 
     'role_hierarchy' => [
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/legacy_encoders.php b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/legacy_encoders.php
index 3c9e6104eecc3..d6206527e6180 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/legacy_encoders.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/php/legacy_encoders.php
@@ -97,7 +97,7 @@
     'access_control' => [
         ['path' => '/blog/524', 'role' => 'ROLE_USER', 'requires_channel' => 'https', 'methods' => ['get', 'POST'], 'port' => 8000],
         ['path' => '/blog/.*', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY'],
-        ['path' => '/blog/524', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY', 'allow_if' => "token.getUsername() matches '/^admin/'"],
+        ['path' => '/blog/524', 'role' => 'IS_AUTHENTICATED_ANONYMOUSLY', 'allow_if' => "token.getUserIdentifier() matches '/^admin/'"],
     ],
 
     'role_hierarchy' => [
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_customized_config.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_customized_config.xml
index b58028b2fbfe3..012c8dac7b069 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_customized_config.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_customized_config.xml
@@ -12,7 +12,7 @@
 
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_default_strategy.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_default_strategy.xml
index 5bffea64f5bf5..1011f45c4accc 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_default_strategy.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_default_strategy.xml
@@ -10,7 +10,7 @@
     <config>
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service.xml
index 9f9f9d5a34e27..ebc208c057168 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service.xml
@@ -12,7 +12,7 @@
 
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service_and_strategy.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service_and_strategy.xml
index 06ee3435e5a7f..1f2133ffe02f1 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service_and_strategy.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/access_decision_manager_service_and_strategy.xml
@@ -12,7 +12,7 @@
 
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/container1.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/container1.xml
index 097a726db58d2..ed7afe5e833ee 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/container1.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/container1.xml
@@ -25,20 +25,20 @@
 
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
         <provider name="digest">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER, ROLE_ADMIN" />
+                <user identifier="foo" password="foo" roles="ROLE_USER, ROLE_ADMIN" />
             </memory>
         </provider>
 
         <provider name="basic">
             <memory>
-                <user name="foo" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_SUPER_ADMIN" />
-                <user name="bar" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_USER, ROLE_ADMIN" />
+                <user identifier="foo" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_SUPER_ADMIN" />
+                <user identifier="bar" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_USER, ROLE_ADMIN" />
             </memory>
         </provider>
 
@@ -78,6 +78,6 @@
 
         <rule path="/blog/524" role="ROLE_USER" requires-channel="https" methods="get,POST" port="8000" />
         <rule role='IS_AUTHENTICATED_ANONYMOUSLY' path="/blog/.*" />
-        <rule role='IS_AUTHENTICATED_ANONYMOUSLY' allow-if="token.getUsername() matches '/^admin/'" path="/blog/524" />
+        <rule role='IS_AUTHENTICATED_ANONYMOUSLY' allow-if="token.getUserIdentifier() matches '/^admin/'" path="/blog/524" />
     </config>
 </srv:container>
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/legacy_encoders.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/legacy_encoders.xml
index 84d68cc4fd59b..a362a59a15b80 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/legacy_encoders.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/legacy_encoders.xml
@@ -25,20 +25,20 @@
 
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
         <provider name="digest">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER, ROLE_ADMIN" />
+                <user identifier="foo" password="foo" roles="ROLE_USER, ROLE_ADMIN" />
             </memory>
         </provider>
 
         <provider name="basic">
             <memory>
-                <user name="foo" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_SUPER_ADMIN" />
-                <user name="bar" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_USER, ROLE_ADMIN" />
+                <user identifier="foo" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_SUPER_ADMIN" />
+                <user identifier="bar" password="0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" roles="ROLE_USER, ROLE_ADMIN" />
             </memory>
         </provider>
 
@@ -78,6 +78,6 @@
 
         <rule path="/blog/524" role="ROLE_USER" requires-channel="https" methods="get,POST" port="8000" />
         <rule role='IS_AUTHENTICATED_ANONYMOUSLY' path="/blog/.*" />
-        <rule role='IS_AUTHENTICATED_ANONYMOUSLY' allow-if="token.getUsername() matches '/^admin/'" path="/blog/524" />
+        <rule role='IS_AUTHENTICATED_ANONYMOUSLY' allow-if="token.getUserIdentifier() matches '/^admin/'" path="/blog/524" />
     </config>
 </srv:container>
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/no_custom_user_checker.xml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/no_custom_user_checker.xml
index 9dd035b7c47e3..3c545ecedc0be 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/no_custom_user_checker.xml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/xml/no_custom_user_checker.xml
@@ -10,7 +10,7 @@
     <config>
         <provider name="default">
             <memory>
-                <user name="foo" password="foo" roles="ROLE_USER" />
+                <user identifier="foo" password="foo" roles="ROLE_USER" />
             </memory>
         </provider>
 
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/container1.yml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/container1.yml
index 0ac2c94b0680b..3eb50b91b7370 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/container1.yml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/container1.yml
@@ -84,4 +84,4 @@ security:
         -
             path: /blog/.*
             role: IS_AUTHENTICATED_ANONYMOUSLY
-        - { path: /blog/524, role: IS_AUTHENTICATED_ANONYMOUSLY, allow_if: "token.getUsername() matches '/^admin/'" }
+        - { path: /blog/524, role: IS_AUTHENTICATED_ANONYMOUSLY, allow_if: "token.getUserIdentifier() matches '/^admin/'" }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/legacy_encoders.yml b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/legacy_encoders.yml
index 03b9aaf6ef5b9..d80a99afcfca3 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/legacy_encoders.yml
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/DependencyInjection/Fixtures/yml/legacy_encoders.yml
@@ -84,4 +84,4 @@ security:
         -
             path: /blog/.*
             role: IS_AUTHENTICATED_ANONYMOUSLY
-        - { path: /blog/524, role: IS_AUTHENTICATED_ANONYMOUSLY, allow_if: "token.getUsername() matches '/^admin/'" }
+        - { path: /blog/524, role: IS_AUTHENTICATED_ANONYMOUSLY, allow_if: "token.getUserIdentifier() matches '/^admin/'" }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/AuthenticatorBundle/ProfileController.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/AuthenticatorBundle/ProfileController.php
index 3e23d86e37483..3e1598ea593cd 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/AuthenticatorBundle/ProfileController.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/AuthenticatorBundle/ProfileController.php
@@ -19,6 +19,6 @@ public function __invoke()
     {
         $this->denyAccessUnlessGranted('ROLE_USER');
 
-        return $this->json(['email' => $this->getUser()->getUsername()]);
+        return $this->json(['email' => $this->getUser()->getUserIdentifier()]);
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/CsrfFormLoginBundle/Resources/views/Login/after_login.html.twig b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/CsrfFormLoginBundle/Resources/views/Login/after_login.html.twig
index a117cb94f8778..9a9bfbc731397 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/CsrfFormLoginBundle/Resources/views/Login/after_login.html.twig
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/CsrfFormLoginBundle/Resources/views/Login/after_login.html.twig
@@ -1,7 +1,7 @@
 {% extends "base.html.twig" %}
 
 {% block body %}
-    Hello {{ app.user.username }}!<br /><br />
+    Hello {{ app.user.userIdentifier }}!<br /><br />
     You're browsing to path "{{ app.request.pathInfo }}".<br /><br />
     <a href="{{ logout_path('default') }}">Log out</a>.
     <a href="{{ logout_url('default') }}">Log out</a>.
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/FormLoginBundle/Resources/views/Login/after_login.html.twig b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/FormLoginBundle/Resources/views/Login/after_login.html.twig
index 3f88aae903536..fd51df2a4383f 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/FormLoginBundle/Resources/views/Login/after_login.html.twig
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/FormLoginBundle/Resources/views/Login/after_login.html.twig
@@ -1,7 +1,7 @@
 {% extends "base.html.twig" %}
 
 {% block body %}
-    Hello {{ user.username }}!<br /><br />
+    Hello {{ user.userIdentifier }}!<br /><br />
     You're browsing to path "{{ app.request.pathInfo }}".
 
     <a href="{{ logout_path('default') }}">Log out</a>.
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/GuardedBundle/AuthenticationController.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/GuardedBundle/AuthenticationController.php
index b8af5cee433a3..21a2ea9e4b8f6 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/GuardedBundle/AuthenticationController.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/GuardedBundle/AuthenticationController.php
@@ -33,6 +33,6 @@ public function profileAction(UserInterface $user = null)
             return new Response('Not logged in.');
         }
 
-        return new Response('Username: '.$user->getUsername());
+        return new Response('Username: '.$user->getUserIdentifier());
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Controller/TestController.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Controller/TestController.php
index cba75a1526ace..6bd571d15e217 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Controller/TestController.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Controller/TestController.php
@@ -21,6 +21,6 @@ class TestController
 {
     public function loginCheckAction(UserInterface $user)
     {
-        return new JsonResponse(['message' => sprintf('Welcome @%s!', $user->getUsername())]);
+        return new JsonResponse(['message' => sprintf('Welcome @%s!', $user->getUserIdentifier())]);
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Security/Http/JsonAuthenticationSuccessHandler.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Security/Http/JsonAuthenticationSuccessHandler.php
index a0300d4d78387..4aabaacd4889c 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Security/Http/JsonAuthenticationSuccessHandler.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/JsonLoginBundle/Security/Http/JsonAuthenticationSuccessHandler.php
@@ -21,6 +21,6 @@ class JsonAuthenticationSuccessHandler implements AuthenticationSuccessHandlerIn
 {
     public function onAuthenticationSuccess(Request $request, TokenInterface $token): Response
     {
-        return new JsonResponse(['message' => sprintf('Good game @%s!', $token->getUsername())]);
+        return new JsonResponse(['message' => sprintf('Good game @%s!', $token->getUserIdentifier())]);
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/LoginLink/TestCustomLoginLinkSuccessHandler.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/LoginLink/TestCustomLoginLinkSuccessHandler.php
index 329a288584a77..a20866c8cfd91 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/LoginLink/TestCustomLoginLinkSuccessHandler.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/LoginLink/TestCustomLoginLinkSuccessHandler.php
@@ -11,6 +11,6 @@ class TestCustomLoginLinkSuccessHandler implements AuthenticationSuccessHandlerI
 {
     public function onAuthenticationSuccess(Request $request, TokenInterface $token)
     {
-        return new JsonResponse(['message' => sprintf('Welcome %s!', $token->getUsername())]);
+        return new JsonResponse(['message' => sprintf('Welcome %s!', $token->getUserIdentifier())]);
     }
 }
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/SecuredPageBundle/Security/Core/User/ArrayUserProvider.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/SecuredPageBundle/Security/Core/User/ArrayUserProvider.php
index 43ea364a525db..a5ca99a41b6b7 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/SecuredPageBundle/Security/Core/User/ArrayUserProvider.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/Bundle/SecuredPageBundle/Security/Core/User/ArrayUserProvider.php
@@ -4,7 +4,7 @@
 
 use Symfony\Bundle\SecurityBundle\Tests\Functional\UserWithoutEquatable;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
@@ -16,7 +16,7 @@ class ArrayUserProvider implements UserProviderInterface
 
     public function addUser(UserInterface $user)
     {
-        $this->users[$user->getUsername()] = $user;
+        $this->users[$user->getUserIdentifier()] = $user;
     }
 
     public function setUser($username, UserInterface $user)
@@ -31,11 +31,16 @@ public function getUser($username)
 
     public function loadUserByUsername($username)
     {
-        $user = $this->getUser($username);
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+        $user = $this->getUser($identifier);
 
         if (null === $user) {
-            $e = new UsernameNotFoundException(sprintf('User "%s" not found.', $username));
-            $e->setUsername($username);
+            $e = new UserNotFoundException(sprintf('User "%s" not found.', $identifier));
+            $e->setUsername($identifier);
 
             throw $e;
         }
@@ -49,10 +54,10 @@ public function refreshUser(UserInterface $user)
             throw new UnsupportedUserException(sprintf('Instances of "%s" are not supported.', get_debug_type($user)));
         }
 
-        $storedUser = $this->getUser($user->getUsername());
+        $storedUser = $this->getUser($user->getUserIdentifier());
         $class = \get_class($storedUser);
 
-        return new $class($storedUser->getUsername(), $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled());
+        return new $class($storedUser->getUserIdentifier(), $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled());
     }
 
     public function supportsClass($class)
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/ClearRememberMeTest.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/ClearRememberMeTest.php
index a917e66c572c9..66a6676375436 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/ClearRememberMeTest.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/ClearRememberMeTest.php
@@ -51,7 +51,7 @@ class RememberMeFooController
 {
     public function __invoke(UserInterface $user)
     {
-        return new Response($user->getUsername());
+        return new Response($user->getUserIdentifier());
     }
 }
 
@@ -66,7 +66,12 @@ public function __construct(InMemoryUserProvider $inner)
 
     public function loadUserByUsername($username)
     {
-        return $this->inner->loadUserByUsername($username);
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+        return $this->inner->loadUserByIdentifier($identifier);
     }
 
     public function refreshUser(UserInterface $user)
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/LoginLinkAuthenticationTest.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/LoginLinkAuthenticationTest.php
index f45ec58055ffe..2c767349287d8 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/LoginLinkAuthenticationTest.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/LoginLinkAuthenticationTest.php
@@ -28,7 +28,7 @@ public function testLoginLinkSuccess()
             $this->markTestSkipped('Login link auth requires symfony/security-http:^5.2');
         }
 
-        $client = $this->createClient(['test_case' => 'LoginLink', 'root_config' => 'config.yml']);
+        $client = $this->createClient(['test_case' => 'LoginLink', 'root_config' => 'config.yml', 'debug' => true]);
 
         // we need an active request that is under the firewall to use the linker
         $request = Request::create('/get-login-link');
diff --git a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/SecurityTest.php b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/SecurityTest.php
index e6f06fa2ccaf3..9aae384648fa7 100644
--- a/src/Symfony/Bundle/SecurityBundle/Tests/Functional/SecurityTest.php
+++ b/src/Symfony/Bundle/SecurityBundle/Tests/Functional/SecurityTest.php
@@ -106,7 +106,7 @@ public function __construct(?string $username, ?string $password, array $roles =
 
     public function __toString()
     {
-        return $this->getUsername();
+        return $this->getUserIdentifier();
     }
 
     /**
@@ -141,6 +141,11 @@ public function getUsername()
         return $this->username;
     }
 
+    public function getUserIdentifier()
+    {
+        return $this->username;
+    }
+
     /**
      * {@inheritdoc}
      */
diff --git a/src/Symfony/Component/Ldap/Security/CheckLdapCredentialsListener.php b/src/Symfony/Component/Ldap/Security/CheckLdapCredentialsListener.php
index e1b79443ac039..e32793c3d3bf4 100644
--- a/src/Symfony/Component/Ldap/Security/CheckLdapCredentialsListener.php
+++ b/src/Symfony/Component/Ldap/Security/CheckLdapCredentialsListener.php
@@ -83,7 +83,8 @@ public function onCheckPassport(CheckPassportEvent $event)
                 } else {
                     throw new LogicException('Using the "query_string" config without using a "search_dn" and a "search_password" is not supported.');
                 }
-                $username = $ldap->escape($user->getUsername(), '', LdapInterface::ESCAPE_FILTER);
+                // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+                $username = $ldap->escape(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(), '', LdapInterface::ESCAPE_FILTER);
                 $query = str_replace('{username}', $username, $ldapBadge->getQueryString());
                 $result = $ldap->query($ldapBadge->getDnString(), $query)->execute();
                 if (1 !== $result->count()) {
@@ -92,7 +93,8 @@ public function onCheckPassport(CheckPassportEvent $event)
 
                 $dn = $result[0]->getDn();
             } else {
-                $username = $ldap->escape($user->getUsername(), '', LdapInterface::ESCAPE_DN);
+                // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+                $username = $ldap->escape(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(), '', LdapInterface::ESCAPE_DN);
                 $dn = str_replace('{username}', $username, $ldapBadge->getDnString());
             }
 
diff --git a/src/Symfony/Component/Ldap/Security/LdapUser.php b/src/Symfony/Component/Ldap/Security/LdapUser.php
index 7c6a2c261c02e..86d6b0c4ad19c 100644
--- a/src/Symfony/Component/Ldap/Security/LdapUser.php
+++ b/src/Symfony/Component/Ldap/Security/LdapUser.php
@@ -75,6 +75,13 @@ public function getSalt(): ?string
      * {@inheritdoc}
      */
     public function getUsername(): string
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated and will be removed in 6.0, use getUserIdentifier() instead.', __METHOD__);
+
+        return $this->username;
+    }
+
+    public function getUserIdentifier(): string
     {
         return $this->username;
     }
diff --git a/src/Symfony/Component/Ldap/Security/LdapUserProvider.php b/src/Symfony/Component/Ldap/Security/LdapUserProvider.php
index 6b0b4304ba8f6..319d6c605c43c 100644
--- a/src/Symfony/Component/Ldap/Security/LdapUserProvider.php
+++ b/src/Symfony/Component/Ldap/Security/LdapUserProvider.php
@@ -17,7 +17,7 @@
 use Symfony\Component\Ldap\LdapInterface;
 use Symfony\Component\Security\Core\Exception\InvalidArgumentException;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
@@ -48,7 +48,7 @@ public function __construct(LdapInterface $ldap, string $baseDn, string $searchD
         }
 
         if (null === $filter) {
-            $filter = '({uid_key}={username})';
+            $filter = '({uid_key}={user_identifier})';
         }
 
         $this->ldap = $ldap;
@@ -66,15 +66,22 @@ public function __construct(LdapInterface $ldap, string $baseDn, string $searchD
      * {@inheritdoc}
      */
     public function loadUserByUsername(string $username)
+    {
+        trigger_deprecation('symfony/ldap', '5.3', 'Method "%s()" is deprecated, use loadUserByIdentifier() instead.', __METHOD__);
+
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
     {
         try {
             $this->ldap->bind($this->searchDn, $this->searchPassword);
-            $username = $this->ldap->escape($username, '', LdapInterface::ESCAPE_FILTER);
-            $query = str_replace('{username}', $username, $this->defaultSearch);
+            $identifier = $this->ldap->escape($identifier, '', LdapInterface::ESCAPE_FILTER);
+            $query = str_replace(['{username}', '{user_identifier}'], $identifier, $this->defaultSearch);
             $search = $this->ldap->query($this->baseDn, $query);
         } catch (ConnectionException $e) {
-            $e = new UsernameNotFoundException(sprintf('User "%s" not found.', $username), 0, $e);
-            $e->setUsername($username);
+            $e = new UserNotFoundException(sprintf('User "%s" not found.', $identifier), 0, $e);
+            $e->setUserIdentifier($identifier);
 
             throw $e;
         }
@@ -83,15 +90,15 @@ public function loadUserByUsername(string $username)
         $count = \count($entries);
 
         if (!$count) {
-            $e = new UsernameNotFoundException(sprintf('User "%s" not found.', $username));
-            $e->setUsername($username);
+            $e = new UserNotFoundException(sprintf('User "%s" not found.', $identifier));
+            $e->setUserIdentifier($identifier);
 
             throw $e;
         }
 
         if ($count > 1) {
-            $e = new UsernameNotFoundException('More than one user found.');
-            $e->setUsername($username);
+            $e = new UserNotFoundException('More than one user found.');
+            $e->setUserIdentifier($identifier);
 
             throw $e;
         }
@@ -100,12 +107,12 @@ public function loadUserByUsername(string $username)
 
         try {
             if (null !== $this->uidKey) {
-                $username = $this->getAttributeValue($entry, $this->uidKey);
+                $identifier = $this->getAttributeValue($entry, $this->uidKey);
             }
         } catch (InvalidArgumentException $e) {
         }
 
-        return $this->loadUser($username, $entry);
+        return $this->loadUser($identifier, $entry);
     }
 
     /**
@@ -117,7 +124,7 @@ public function refreshUser(UserInterface $user)
             throw new UnsupportedUserException(sprintf('Instances of "%s" are not supported.', get_debug_type($user)));
         }
 
-        return new LdapUser($user->getEntry(), $user->getUsername(), $user->getPassword(), $user->getRoles(), $user->getExtraFields());
+        return new LdapUser($user->getEntry(), $user->getUserIdentifier(), $user->getPassword(), $user->getRoles(), $user->getExtraFields());
     }
 
     /**
@@ -157,7 +164,7 @@ public function supportsClass(string $class)
      *
      * @return UserInterface
      */
-    protected function loadUser(string $username, Entry $entry)
+    protected function loadUser(string $identifier, Entry $entry)
     {
         $password = null;
         $extraFields = [];
@@ -170,7 +177,7 @@ protected function loadUser(string $username, Entry $entry)
             $extraFields[$field] = $this->getAttributeValue($entry, $field);
         }
 
-        return new LdapUser($entry, $username, $password, $this->defaultRoles, $extraFields);
+        return new LdapUser($entry, $identifier, $password, $this->defaultRoles, $extraFields);
     }
 
     private function getAttributeValue(Entry $entry, string $attribute)
diff --git a/src/Symfony/Component/Ldap/Tests/Security/LdapUserProviderTest.php b/src/Symfony/Component/Ldap/Tests/Security/LdapUserProviderTest.php
index f6cd4c7414c51..32f6d60d5df3b 100644
--- a/src/Symfony/Component/Ldap/Tests/Security/LdapUserProviderTest.php
+++ b/src/Symfony/Component/Ldap/Tests/Security/LdapUserProviderTest.php
@@ -20,7 +20,7 @@
 use Symfony\Component\Ldap\Security\LdapUser;
 use Symfony\Component\Ldap\Security\LdapUserProvider;
 use Symfony\Component\Security\Core\Exception\InvalidArgumentException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 
 /**
  * @requires extension ldap
@@ -29,7 +29,7 @@ class LdapUserProviderTest extends TestCase
 {
     public function testLoadUserByUsernameFailsIfCantConnectToLdap()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
 
         $ldap = $this->createMock(LdapInterface::class);
         $ldap
@@ -39,12 +39,12 @@ public function testLoadUserByUsernameFailsIfCantConnectToLdap()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com');
-        $provider->loadUserByUsername('foo');
+        $provider->loadUserByIdentifier('foo');
     }
 
     public function testLoadUserByUsernameFailsIfNoLdapEntries()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
 
         $result = $this->createMock(CollectionInterface::class);
         $query = $this->createMock(QueryInterface::class);
@@ -71,12 +71,12 @@ public function testLoadUserByUsernameFailsIfNoLdapEntries()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com');
-        $provider->loadUserByUsername('foo');
+        $provider->loadUserByIdentifier('foo');
     }
 
     public function testLoadUserByUsernameFailsIfMoreThanOneLdapEntry()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
 
         $result = $this->createMock(CollectionInterface::class);
         $query = $this->createMock(QueryInterface::class);
@@ -103,7 +103,7 @@ public function testLoadUserByUsernameFailsIfMoreThanOneLdapEntry()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com');
-        $provider->loadUserByUsername('foo');
+        $provider->loadUserByIdentifier('foo');
     }
 
     public function testLoadUserByUsernameFailsIfMoreThanOneLdapPasswordsInEntry()
@@ -144,7 +144,7 @@ public function testLoadUserByUsernameFailsIfMoreThanOneLdapPasswordsInEntry()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com', null, null, [], 'sAMAccountName', '({uid_key}={username})', 'userpassword');
-        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByUsername('foo'));
+        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByIdentifier('foo'));
     }
 
     public function testLoadUserByUsernameShouldNotFailIfEntryHasNoUidKeyAttribute()
@@ -180,7 +180,7 @@ public function testLoadUserByUsernameShouldNotFailIfEntryHasNoUidKeyAttribute()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com', null, null, [], 'sAMAccountName', '({uid_key}={username})');
-        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByUsername('foo'));
+        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByIdentifier('foo'));
     }
 
     public function testLoadUserByUsernameFailsIfEntryHasNoPasswordAttribute()
@@ -218,7 +218,7 @@ public function testLoadUserByUsernameFailsIfEntryHasNoPasswordAttribute()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com', null, null, [], 'sAMAccountName', '({uid_key}={username})', 'userpassword');
-        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByUsername('foo'));
+        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByIdentifier('foo'));
     }
 
     public function testLoadUserByUsernameIsSuccessfulWithoutPasswordAttribute()
@@ -254,7 +254,7 @@ public function testLoadUserByUsernameIsSuccessfulWithoutPasswordAttribute()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com');
-        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByUsername('foo'));
+        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByIdentifier('foo'));
     }
 
     public function testLoadUserByUsernameIsSuccessfulWithoutPasswordAttributeAndWrongCase()
@@ -290,7 +290,7 @@ public function testLoadUserByUsernameIsSuccessfulWithoutPasswordAttributeAndWro
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com');
-        $this->assertSame('foo', $provider->loadUserByUsername('Foo')->getUsername());
+        $this->assertSame('foo', $provider->loadUserByIdentifier('Foo')->getUserIdentifier());
     }
 
     public function testLoadUserByUsernameIsSuccessfulWithPasswordAttribute()
@@ -330,7 +330,7 @@ public function testLoadUserByUsernameIsSuccessfulWithPasswordAttribute()
         ;
 
         $provider = new LdapUserProvider($ldap, 'ou=MyBusiness,dc=symfony,dc=com', null, null, [], 'sAMAccountName', '({uid_key}={username})', 'userpassword', ['email']);
-        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByUsername('foo'));
+        $this->assertInstanceOf(LdapUser::class, $provider->loadUserByIdentifier('foo'));
     }
 
     public function testRefreshUserShouldReturnUserWithSameProperties()
diff --git a/src/Symfony/Component/PasswordHasher/Tests/Hasher/PasswordHasherFactoryTest.php b/src/Symfony/Component/PasswordHasher/Tests/Hasher/PasswordHasherFactoryTest.php
index 46d430f51a8c4..633e38e39eef0 100644
--- a/src/Symfony/Component/PasswordHasher/Tests/Hasher/PasswordHasherFactoryTest.php
+++ b/src/Symfony/Component/PasswordHasher/Tests/Hasher/PasswordHasherFactoryTest.php
@@ -196,6 +196,10 @@ public function getUsername(): string
     {
     }
 
+    public function getUserIdentifier(): string
+    {
+    }
+
     public function eraseCredentials()
     {
     }
diff --git a/src/Symfony/Component/Security/CHANGELOG.md b/src/Symfony/Component/Security/CHANGELOG.md
index d3f7ca0322823..adfd240992712 100644
--- a/src/Symfony/Component/Security/CHANGELOG.md
+++ b/src/Symfony/Component/Security/CHANGELOG.md
@@ -4,6 +4,11 @@ CHANGELOG
 5.3
 ---
 
+ * Deprecate `PersistentTokenInterface::getUsername()` in favor of `PersistentTokenInterface::getUserIdentifier()`
+ * Deprecate `UsernameNotFoundException` in favor of `UserNotFoundException` and `getUsername()`/`setUsername()` in favor of `getUserIdentifier()`/`setUserIdentifier()`
+ * Deprecate `UserProviderInterface::loadUserByUsername()` in favor of `UserProviderInterface::loadUserByIdentifier()`
+ * Deprecate `TokenInterface::getUsername()` in favor of `TokenInterface::getUserIdentifier()`
+ * Deprecate `UserInterface::getUsername()` in favor of `getUserIdentifier()`
  * Add `PassportInterface:getBadges()`, implemented by `PassportTrait`
  * [BC BREAK] Remove method `checkIfCompletelyResolved()` from `PassportInterface`, checking that passport badges are
    resolved is up to `AuthenticatorManager`
diff --git a/src/Symfony/Component/Security/Core/Authentication/AuthenticationProviderManager.php b/src/Symfony/Component/Security/Core/Authentication/AuthenticationProviderManager.php
index c4099603ef59f..ddf098306d4b3 100644
--- a/src/Symfony/Component/Security/Core/Authentication/AuthenticationProviderManager.php
+++ b/src/Symfony/Component/Security/Core/Authentication/AuthenticationProviderManager.php
@@ -21,6 +21,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\Exception\ProviderNotFoundException;
+use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
 
 // Help opcache.preload discover always-needed symbols
@@ -105,6 +106,11 @@ public function authenticate(TokenInterface $token)
                 $this->eventDispatcher->dispatch(new AuthenticationSuccessEvent($result), AuthenticationEvents::AUTHENTICATION_SUCCESS);
             }
 
+            // @deprecated since 5.3
+            if ($user = $result->getUser() instanceof UserInterface && !method_exists($result->getUser(), 'getUserIdentifier')) {
+                trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "getUserIdentifier(): string" in user class "%s" is deprecated. This method will replace "getUsername()" in Symfony 6.0.', get_debug_type($result->getUser()));
+            }
+
             return $result;
         }
 
diff --git a/src/Symfony/Component/Security/Core/Authentication/Provider/DaoAuthenticationProvider.php b/src/Symfony/Component/Security/Core/Authentication/Provider/DaoAuthenticationProvider.php
index eca9357f63a67..4ef55664dc513 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Provider/DaoAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Provider/DaoAuthenticationProvider.php
@@ -16,7 +16,7 @@
 use Symfony\Component\Security\Core\Encoder\EncoderFactoryInterface;
 use Symfony\Component\Security\Core\Exception\AuthenticationServiceException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\LegacyPasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
@@ -108,7 +108,7 @@ protected function checkAuthentication(UserInterface $user, UsernamePasswordToke
     /**
      * {@inheritdoc}
      */
-    protected function retrieveUser(string $username, UsernamePasswordToken $token)
+    protected function retrieveUser(string $userIdentifier, UsernamePasswordToken $token)
     {
         $user = $token->getUser();
         if ($user instanceof UserInterface) {
@@ -116,15 +116,22 @@ protected function retrieveUser(string $username, UsernamePasswordToken $token)
         }
 
         try {
-            $user = $this->userProvider->loadUserByUsername($username);
+            // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+            if (method_exists($this->userProvider, 'loadUserByIdentifier')) {
+                $user = $this->userProvider->loadUserByIdentifier($userIdentifier);
+            } else {
+                trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+                $user = $this->userProvider->loadUserByUsername($userIdentifier);
+            }
 
             if (!$user instanceof UserInterface) {
                 throw new AuthenticationServiceException('The user provider must return a UserInterface object.');
             }
 
             return $user;
-        } catch (UsernameNotFoundException $e) {
-            $e->setUsername($username);
+        } catch (UserNotFoundException $e) {
+            $e->setUserIdentifier($userIdentifier);
             throw $e;
         } catch (\Exception $e) {
             $e = new AuthenticationServiceException($e->getMessage(), 0, $e);
diff --git a/src/Symfony/Component/Security/Core/Authentication/Provider/LdapBindAuthenticationProvider.php b/src/Symfony/Component/Security/Core/Authentication/Provider/LdapBindAuthenticationProvider.php
index 3705ae8289a19..e9a3ab029413f 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Provider/LdapBindAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Provider/LdapBindAuthenticationProvider.php
@@ -16,7 +16,7 @@
 use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\Exception\LogicException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
@@ -38,7 +38,7 @@ class LdapBindAuthenticationProvider extends UserAuthenticationProvider
     private $searchDn;
     private $searchPassword;
 
-    public function __construct(UserProviderInterface $userProvider, UserCheckerInterface $userChecker, string $providerKey, LdapInterface $ldap, string $dnString = '{username}', bool $hideUserNotFoundExceptions = true, string $searchDn = '', string $searchPassword = '')
+    public function __construct(UserProviderInterface $userProvider, UserCheckerInterface $userChecker, string $providerKey, LdapInterface $ldap, string $dnString = '{user_identifier}', bool $hideUserNotFoundExceptions = true, string $searchDn = '', string $searchPassword = '')
     {
         parent::__construct($userChecker, $providerKey, $hideUserNotFoundExceptions);
 
@@ -50,7 +50,7 @@ public function __construct(UserProviderInterface $userProvider, UserCheckerInte
     }
 
     /**
-     * Set a query string to use in order to find a DN for the username.
+     * Set a query string to use in order to find a DN for the user identifier.
      */
     public function setQueryString(string $queryString)
     {
@@ -60,13 +60,20 @@ public function setQueryString(string $queryString)
     /**
      * {@inheritdoc}
      */
-    protected function retrieveUser(string $username, UsernamePasswordToken $token)
+    protected function retrieveUser(string $userIdentifier, UsernamePasswordToken $token)
     {
-        if (AuthenticationProviderInterface::USERNAME_NONE_PROVIDED === $username) {
-            throw new UsernameNotFoundException('Username can not be null.');
+        if (AuthenticationProviderInterface::USERNAME_NONE_PROVIDED === $userIdentifier) {
+            throw new UserNotFoundException('User identifier can not be null.');
         }
 
-        return $this->userProvider->loadUserByUsername($username);
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        if (method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            return $this->userProvider->loadUserByIdentifier($userIdentifier);
+        } else {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            return $this->userProvider->loadUserByUsername($userIdentifier);
+        }
     }
 
     /**
@@ -74,7 +81,8 @@ protected function retrieveUser(string $username, UsernamePasswordToken $token)
      */
     protected function checkAuthentication(UserInterface $user, UsernamePasswordToken $token)
     {
-        $username = $token->getUsername();
+        // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+        $userIdentifier = method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername();
         $password = $token->getCredentials();
 
         if ('' === (string) $password) {
@@ -88,8 +96,8 @@ protected function checkAuthentication(UserInterface $user, UsernamePasswordToke
                 } else {
                     throw new LogicException('Using the "query_string" config without using a "search_dn" and a "search_password" is not supported.');
                 }
-                $username = $this->ldap->escape($username, '', LdapInterface::ESCAPE_FILTER);
-                $query = str_replace('{username}', $username, $this->queryString);
+                $userIdentifier = $this->ldap->escape($userIdentifier, '', LdapInterface::ESCAPE_FILTER);
+                $query = str_replace(['{username}', '{user_identifier}'], $userIdentifier, $this->queryString);
                 $result = $this->ldap->query($this->dnString, $query)->execute();
                 if (1 !== $result->count()) {
                     throw new BadCredentialsException('The presented username is invalid.');
@@ -97,8 +105,8 @@ protected function checkAuthentication(UserInterface $user, UsernamePasswordToke
 
                 $dn = $result[0]->getDn();
             } else {
-                $username = $this->ldap->escape($username, '', LdapInterface::ESCAPE_DN);
-                $dn = str_replace('{username}', $username, $this->dnString);
+                $userIdentifier = $this->ldap->escape($userIdentifier, '', LdapInterface::ESCAPE_DN);
+                $dn = str_replace(['{username}', '{user_identifier}'], $userIdentifier, $this->dnString);
             }
 
             $this->ldap->bind($dn, $password);
diff --git a/src/Symfony/Component/Security/Core/Authentication/Provider/PreAuthenticatedAuthenticationProvider.php b/src/Symfony/Component/Security/Core/Authentication/Provider/PreAuthenticatedAuthenticationProvider.php
index c0612bc0b61c4..292b8b9fed0f2 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Provider/PreAuthenticatedAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Provider/PreAuthenticatedAuthenticationProvider.php
@@ -24,7 +24,7 @@
  * This authentication provider will not perform any checks on authentication
  * requests, as they should already be pre-authenticated. However, the
  * UserProviderInterface implementation may still throw a
- * UsernameNotFoundException, for example.
+ * UserNotFoundException, for example.
  *
  * @author Fabien Potencier <fabien@symfony.com>
  */
@@ -54,7 +54,15 @@ public function authenticate(TokenInterface $token)
             throw new BadCredentialsException('No pre-authenticated principal found in request.');
         }
 
-        $user = $this->userProvider->loadUserByUsername($user);
+        $userIdentifier = method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername();
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        if (method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            $user = $this->userProvider->loadUserByIdentifier($userIdentifier);
+        } else {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $user = $this->userProvider->loadUserByUsername($userIdentifier);
+        }
 
         $this->userChecker->checkPostAuth($user);
 
diff --git a/src/Symfony/Component/Security/Core/Authentication/Provider/RememberMeAuthenticationProvider.php b/src/Symfony/Component/Security/Core/Authentication/Provider/RememberMeAuthenticationProvider.php
index 630064af447ba..8ee8109b80170 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Provider/RememberMeAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Provider/RememberMeAuthenticationProvider.php
@@ -51,7 +51,7 @@ public function authenticate(TokenInterface $token)
 
         $user = $token->getUser();
 
-        if (!$token->getUser() instanceof UserInterface) {
+        if (!$user instanceof UserInterface) {
             throw new LogicException(sprintf('Method "%s::getUser()" must return a "%s" instance, "%s" returned.', get_debug_type($token), UserInterface::class, get_debug_type($user)));
         }
 
diff --git a/src/Symfony/Component/Security/Core/Authentication/Provider/UserAuthenticationProvider.php b/src/Symfony/Component/Security/Core/Authentication/Provider/UserAuthenticationProvider.php
index 21c1787ea9d52..4dfff685a7706 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Provider/UserAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Provider/UserAuthenticationProvider.php
@@ -17,7 +17,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\AuthenticationServiceException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
 
@@ -55,18 +55,18 @@ public function authenticate(TokenInterface $token)
             throw new AuthenticationException('The token is not supported by this authentication provider.');
         }
 
-        $username = $token->getUsername();
+        $username = method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername();
         if ('' === $username || null === $username) {
             $username = AuthenticationProviderInterface::USERNAME_NONE_PROVIDED;
         }
 
         try {
             $user = $this->retrieveUser($username, $token);
-        } catch (UsernameNotFoundException $e) {
+        } catch (UserNotFoundException $e) {
             if ($this->hideUserNotFoundExceptions) {
                 throw new BadCredentialsException('Bad credentials.', 0, $e);
             }
-            $e->setUsername($username);
+            $e->setUserIdentifier($username);
 
             throw $e;
         }
diff --git a/src/Symfony/Component/Security/Core/Authentication/RememberMe/InMemoryTokenProvider.php b/src/Symfony/Component/Security/Core/Authentication/RememberMe/InMemoryTokenProvider.php
index a1b30443f3573..571bbe02ef716 100644
--- a/src/Symfony/Component/Security/Core/Authentication/RememberMe/InMemoryTokenProvider.php
+++ b/src/Symfony/Component/Security/Core/Authentication/RememberMe/InMemoryTokenProvider.php
@@ -45,7 +45,7 @@ public function updateToken(string $series, string $tokenValue, \DateTime $lastU
 
         $token = new PersistentToken(
             $this->tokens[$series]->getClass(),
-            $this->tokens[$series]->getUsername(),
+            method_exists($this->tokens[$series], 'getUserIdentifier') ? $this->tokens[$series]->getUserIdentifier() : $this->tokens[$series]->getUsername(),
             $series,
             $tokenValue,
             $lastUsed
diff --git a/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentToken.php b/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentToken.php
index 1f0e485c50ef2..b8337adff57f3 100644
--- a/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentToken.php
+++ b/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentToken.php
@@ -19,18 +19,18 @@
 final class PersistentToken implements PersistentTokenInterface
 {
     private $class;
-    private $username;
+    private $userIdentifier;
     private $series;
     private $tokenValue;
     private $lastUsed;
 
-    public function __construct(string $class, string $username, string $series, string $tokenValue, \DateTime $lastUsed)
+    public function __construct(string $class, string $userIdentifier, string $series, string $tokenValue, \DateTime $lastUsed)
     {
         if (empty($class)) {
             throw new \InvalidArgumentException('$class must not be empty.');
         }
-        if ('' === $username) {
-            throw new \InvalidArgumentException('$username must not be empty.');
+        if ('' === $userIdentifier) {
+            throw new \InvalidArgumentException('$userIdentifier must not be empty.');
         }
         if (empty($series)) {
             throw new \InvalidArgumentException('$series must not be empty.');
@@ -40,7 +40,7 @@ public function __construct(string $class, string $username, string $series, str
         }
 
         $this->class = $class;
-        $this->username = $username;
+        $this->userIdentifier = $userIdentifier;
         $this->series = $series;
         $this->tokenValue = $tokenValue;
         $this->lastUsed = $lastUsed;
@@ -59,7 +59,14 @@ public function getClass(): string
      */
     public function getUsername(): string
     {
-        return $this->username;
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        return $this->userIdentifier;
+    }
+
+    public function getUserIdentifier(): string
+    {
+        return $this->userIdentifier;
     }
 
     /**
diff --git a/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentTokenInterface.php b/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentTokenInterface.php
index ba31ffa6d022e..85c5bc385e580 100644
--- a/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentTokenInterface.php
+++ b/src/Symfony/Component/Security/Core/Authentication/RememberMe/PersistentTokenInterface.php
@@ -15,6 +15,8 @@
  * Interface to be implemented by persistent token classes (such as
  * Doctrine entities representing a remember-me token).
  *
+ * @method string getUserIdentifier() returns the identifier used to authenticate (e.g. their e-mailaddress or username)
+ *
  * @author Johannes M. Schmitt <schmittjoh@gmail.com>
  */
 interface PersistentTokenInterface
@@ -26,13 +28,6 @@ interface PersistentTokenInterface
      */
     public function getClass();
 
-    /**
-     * Returns the username.
-     *
-     * @return string
-     */
-    public function getUsername();
-
     /**
      * Returns the series.
      *
diff --git a/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php b/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
index 0083ae3957728..b7934137e607f 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php
@@ -51,10 +51,32 @@ public function getRoleNames(): array
     /**
      * {@inheritdoc}
      */
-    public function getUsername()
+    public function getUsername(/* $legacy = true */)
     {
+        if (1 === func_num_args() && false === func_get_arg(0)) {
+            return null;
+        }
+
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        if ($this->user instanceof UserInterface) {
+            return method_exists($this->user, 'getUserIdentifier') ? $this->user->getUserIdentifier() : $this->user->getUsername();
+        }
+
+        return (string) $this->user;
+    }
+
+    public function getUserIdentifier(): string
+    {
+        // method returns "null" in non-legacy mode if not overriden
+        $username = $this->getUsername(false);
+        if (null !== $username) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Method "%s::getUsername()" is deprecated, override "getUserIdentifier()" instead.', get_debug_type($this));
+        }
+
         if ($this->user instanceof UserInterface) {
-            return $this->user->getUsername();
+            // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+            return method_exists($this->user, 'getUserIdentifier') ? $this->user->getUserIdentifier() : $this->user->getUsername();
         }
 
         return (string) $this->user;
@@ -234,7 +256,7 @@ public function __toString()
             $roles[] = $role;
         }
 
-        return sprintf('%s(user="%s", authenticated=%s, roles="%s")', $class, $this->getUsername(), json_encode($this->authenticated), implode(', ', $roles));
+        return sprintf('%s(user="%s", authenticated=%s, roles="%s")', $class, $this->getUserIdentifier(), json_encode($this->authenticated), implode(', ', $roles));
     }
 
     /**
@@ -283,7 +305,11 @@ private function hasUserChanged(UserInterface $user): bool
             return true;
         }
 
-        if ($this->user->getUsername() !== $user->getUsername()) {
+        // @deprecated since Symfony 5.3, drop getUsername() in 6.0
+        $userIdentifier = function ($user) {
+            return method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername();
+        };
+        if ($userIdentifier($this->user) !== $userIdentifier($user)) {
             return true;
         }
 
diff --git a/src/Symfony/Component/Security/Core/Authentication/Token/NullToken.php b/src/Symfony/Component/Security/Core/Authentication/Token/NullToken.php
index 589ad1b47f9c7..5c8a1c24b1e12 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Token/NullToken.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Token/NullToken.php
@@ -42,6 +42,13 @@ public function setUser($user)
     }
 
     public function getUsername()
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        return '';
+    }
+
+    public function getUserIdentifier(): string
     {
         return '';
     }
diff --git a/src/Symfony/Component/Security/Core/Authentication/Token/Storage/TokenStorage.php b/src/Symfony/Component/Security/Core/Authentication/Token/Storage/TokenStorage.php
index 850c05e752672..1fc30bfcd1473 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Token/Storage/TokenStorage.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Token/Storage/TokenStorage.php
@@ -48,6 +48,11 @@ public function setToken(TokenInterface $token = null)
         if ($token) {
             // ensure any initializer is called
             $this->getToken();
+
+            // @deprecated since 5.3
+            if (!method_exists($token, 'getUserIdentifier')) {
+                trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "getUserIdentifier(): string" in token class "%s" is deprecated. This method will replace "getUsername()" in Symfony 6.0.', get_debug_type($token));
+            }
         }
 
         $this->initializer = null;
diff --git a/src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php b/src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php
index ad48ec6455f6a..047f571ae4a20 100644
--- a/src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php
+++ b/src/Symfony/Component/Security/Core/Authentication/Token/TokenInterface.php
@@ -16,6 +16,8 @@
 /**
  * TokenInterface is the interface for the user authentication information.
  *
+ * @method string getUserIdentifier() returns the user identifier used during authentication (e.g. a user's e-mailaddress or username)
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  * @author Johannes M. Schmitt <schmittjoh@gmail.com>
  */
@@ -65,13 +67,6 @@ public function getUser();
      */
     public function setUser($user);
 
-    /**
-     * Returns the username.
-     *
-     * @return string
-     */
-    public function getUsername();
-
     /**
      * Returns whether the user is authenticated or not.
      *
diff --git a/src/Symfony/Component/Security/Core/Exception/UserNotFoundException.php b/src/Symfony/Component/Security/Core/Exception/UserNotFoundException.php
new file mode 100644
index 0000000000000..d730f7d7719f5
--- /dev/null
+++ b/src/Symfony/Component/Security/Core/Exception/UserNotFoundException.php
@@ -0,0 +1,96 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Security\Core\Exception;
+
+/**
+ * UserNotFoundException is thrown if a User cannot be found for the given identifier.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ * @author Alexander <iam.asm89@gmail.com>
+ */
+class UserNotFoundException extends AuthenticationException
+{
+    private $identifier;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getMessageKey()
+    {
+        return 'Username could not be found.';
+    }
+
+    /**
+     * Get the user identifier (e.g. username or e-mailaddress).
+     */
+    public function getUserIdentifier(): string
+    {
+        return $this->identifier;
+    }
+
+    /**
+     * @return string
+     *
+     * @deprecated
+     */
+    public function getUsername()
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        return $this->identifier;
+    }
+
+    /**
+     * Set the user identifier (e.g. username or e-mailaddress).
+     */
+    public function setUserIdentifier(string $identifier): void
+    {
+        $this->identifier = $identifier;
+    }
+
+    /**
+     * @deprecated
+     */
+    public function setUsername(string $username)
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        $this->identifier = $username;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getMessageData()
+    {
+        return ['{{ username }}' => $this->identifier, '{{ user_identifier }}' => $this->identifier];
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __serialize(): array
+    {
+        return [$this->identifier, parent::__serialize()];
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __unserialize(array $data): void
+    {
+        [$this->identifier, $parentData] = $data;
+        $parentData = \is_array($parentData) ? $parentData : unserialize($parentData);
+        parent::__unserialize($parentData);
+    }
+}
+class_alias(UserNotFoundException::class, UsernameNotFoundException::class);
diff --git a/src/Symfony/Component/Security/Core/Exception/UsernameNotFoundException.php b/src/Symfony/Component/Security/Core/Exception/UsernameNotFoundException.php
index f46013236c12a..e0d2d4a2ef113 100644
--- a/src/Symfony/Component/Security/Core/Exception/UsernameNotFoundException.php
+++ b/src/Symfony/Component/Security/Core/Exception/UsernameNotFoundException.php
@@ -11,65 +11,15 @@
 
 namespace Symfony\Component\Security\Core\Exception;
 
-/**
- * UsernameNotFoundException is thrown if a User cannot be found by its username.
- *
- * @author Fabien Potencier <fabien@symfony.com>
- * @author Alexander <iam.asm89@gmail.com>
- */
-class UsernameNotFoundException extends AuthenticationException
-{
-    private $username;
-
-    /**
-     * {@inheritdoc}
-     */
-    public function getMessageKey()
-    {
-        return 'Username could not be found.';
-    }
-
-    /**
-     * Get the username.
-     *
-     * @return string
-     */
-    public function getUsername()
-    {
-        return $this->username;
-    }
-
-    /**
-     * Set the username.
-     */
-    public function setUsername(string $username)
-    {
-        $this->username = $username;
-    }
+trigger_deprecation('symfony/security-core', '5.3', 'The "%s" class is deprecated, use "%s" instead.', UsernameNotFoundException::class, UserNotFoundException::class);
 
-    /**
-     * {@inheritdoc}
-     */
-    public function getMessageData()
-    {
-        return ['{{ username }}' => $this->username];
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function __serialize(): array
-    {
-        return [$this->username, parent::__serialize()];
-    }
+class_exists(UserNotFoundException::class);
 
+if (false) {
     /**
-     * {@inheritdoc}
+     * @deprecated since Symfony 5.3 to be removed in 6.0, use UserNotFoundException instead.
      */
-    public function __unserialize(array $data): void
+    class UsernameNotFoundException extends AuthenticationException
     {
-        [$this->username, $parentData] = $data;
-        $parentData = \is_array($parentData) ? $parentData : unserialize($parentData);
-        parent::__unserialize($parentData);
     }
 }
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationProviderManagerTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationProviderManagerTest.php
index db1e388703bb6..d41805bffb371 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationProviderManagerTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationProviderManagerTest.php
@@ -23,6 +23,7 @@
 use Symfony\Component\Security\Core\Exception\AccountStatusException;
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\ProviderNotFoundException;
+use Symfony\Component\Security\Core\User\InMemoryUser;
 
 class AuthenticationProviderManagerTest extends TestCase
 {
@@ -90,9 +91,12 @@ public function testAuthenticateWhenProviderReturnsAuthenticationException()
 
     public function testAuthenticateWhenOneReturnsAuthenticationExceptionButNotAll()
     {
+        $expected = $this->createMock(TokenInterface::class);
+        $expected->expects($this->any())->method('getUser')->willReturn(new InMemoryUser('wouter', null));
+
         $manager = new AuthenticationProviderManager([
             $this->getAuthenticationProvider(true, null, AuthenticationException::class),
-            $this->getAuthenticationProvider(true, $expected = $this->createMock(TokenInterface::class)),
+            $this->getAuthenticationProvider(true, $expected),
         ]);
 
         $token = $manager->authenticate($this->createMock(TokenInterface::class));
@@ -106,8 +110,10 @@ public function testAuthenticateReturnsTokenOfTheFirstMatchingProvider()
             ->expects($this->never())
             ->method('supports')
         ;
+        $expected = $this->createMock(TokenInterface::class);
+        $expected->expects($this->any())->method('getUser')->willReturn(new InMemoryUser('wouter', null));
         $manager = new AuthenticationProviderManager([
-            $this->getAuthenticationProvider(true, $expected = $this->createMock(TokenInterface::class)),
+            $this->getAuthenticationProvider(true, $expected),
             $second,
         ]);
 
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
index cd1924c63edf6..adb14975bc949 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/AuthenticationTrustResolverTest.php
@@ -155,6 +155,10 @@ public function getUsername(): string
     {
     }
 
+    public function getUserIdentifier(): string
+    {
+    }
+
     public function isAuthenticated(): bool
     {
     }
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/DaoAuthenticationProviderTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/DaoAuthenticationProviderTest.php
index 46b5624bb215e..05340afa8537d 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/DaoAuthenticationProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/DaoAuthenticationProviderTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Security\Core\Tests\Authentication\Provider;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\PasswordHasher\Hasher\PasswordHasherFactoryInterface;
 use Symfony\Component\PasswordHasher\Hasher\PlaintextPasswordHasher;
 use Symfony\Component\PasswordHasher\PasswordHasherInterface;
@@ -20,8 +21,9 @@
 use Symfony\Component\Security\Core\Encoder\EncoderFactoryInterface;
 use Symfony\Component\Security\Core\Exception\AuthenticationServiceException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
+use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -29,13 +31,25 @@
 
 class DaoAuthenticationProviderTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
+    /**
+     * @group legacy
+     */
     public function testRetrieveUserWhenProviderDoesNotReturnAnUserInterface()
     {
         $this->expectException(AuthenticationServiceException::class);
-        $provider = $this->getProvider('fabien');
+        $userProvider = $this->createMock(DaoAuthenticationProviderTest_UserProvider::class);
+        $userProvider->expects($this->once())
+                     ->method('loadUserByUsername')
+                     ->willReturn('fabien')
+        ;
+        $provider = $this->getProvider(null, null, null, $userProvider);
         $method = new \ReflectionMethod($provider, 'retrieveUser');
         $method->setAccessible(true);
 
+        $this->expectDeprecation('Since symfony/security-core 5.3: Not implementing method "loadUserByIdentifier()" in user provider "'.get_debug_type($userProvider).'" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.');
+
         $method->invoke($provider, 'fabien', $this->getSupportedToken());
     }
 
@@ -44,12 +58,8 @@ public function testRetrieveUserWhenProviderDoesNotReturnAnUserInterface()
      */
     public function testRetrieveUserWhenUsernameIsNotFoundWithLegacyEncoderFactory()
     {
-        $this->expectException(UsernameNotFoundException::class);
-        $userProvider = $this->createMock(UserProviderInterface::class);
-        $userProvider->expects($this->once())
-                     ->method('loadUserByUsername')
-                     ->willThrowException(new UsernameNotFoundException())
-        ;
+        $this->expectException(UserNotFoundException::class);
+        $userProvider = new InMemoryUserProvider();
 
         $provider = new DaoAuthenticationProvider($userProvider, $this->createMock(UserCheckerInterface::class), 'key', $this->createMock(EncoderFactoryInterface::class));
         $method = new \ReflectionMethod($provider, 'retrieveUser');
@@ -60,12 +70,8 @@ public function testRetrieveUserWhenUsernameIsNotFoundWithLegacyEncoderFactory()
 
     public function testRetrieveUserWhenUsernameIsNotFound()
     {
-        $this->expectException(UsernameNotFoundException::class);
-        $userProvider = $this->createMock(UserProviderInterface::class);
-        $userProvider->expects($this->once())
-            ->method('loadUserByUsername')
-            ->willThrowException(new UsernameNotFoundException())
-        ;
+        $this->expectException(UserNotFoundException::class);
+        $userProvider = new InMemoryUserProvider();
 
         $provider = new DaoAuthenticationProvider($userProvider, $this->createMock(UserCheckerInterface::class), 'key', $this->createMock(PasswordHasherFactoryInterface::class));
         $method = new \ReflectionMethod($provider, 'retrieveUser');
@@ -77,9 +83,9 @@ public function testRetrieveUserWhenUsernameIsNotFound()
     public function testRetrieveUserWhenAnExceptionOccurs()
     {
         $this->expectException(AuthenticationServiceException::class);
-        $userProvider = $this->createMock(UserProviderInterface::class);
+        $userProvider = $this->createMock(InMemoryUserProvider::class);
         $userProvider->expects($this->once())
-                     ->method('loadUserByUsername')
+                     ->method('loadUserByIdentifier')
                      ->willThrowException(new \RuntimeException())
         ;
 
@@ -92,9 +98,9 @@ public function testRetrieveUserWhenAnExceptionOccurs()
 
     public function testRetrieveUserReturnsUserFromTokenOnReauthentication()
     {
-        $userProvider = $this->createMock(UserProviderInterface::class);
+        $userProvider = $this->createMock(InMemoryUserProvider::class);
         $userProvider->expects($this->never())
-                     ->method('loadUserByUsername')
+                     ->method('loadUserByIdentifier')
         ;
 
         $user = new TestUser();
@@ -114,19 +120,13 @@ public function testRetrieveUserReturnsUserFromTokenOnReauthentication()
 
     public function testRetrieveUser()
     {
-        $user = new TestUser();
-
-        $userProvider = $this->createMock(UserProviderInterface::class);
-        $userProvider->expects($this->once())
-                     ->method('loadUserByUsername')
-                     ->willReturn($user)
-        ;
+        $userProvider = new InMemoryUserProvider(['fabien' => []]);
 
         $provider = new DaoAuthenticationProvider($userProvider, $this->createMock(UserCheckerInterface::class), 'key', $this->createMock(PasswordHasherFactoryInterface::class));
         $method = new \ReflectionMethod($provider, 'retrieveUser');
         $method->setAccessible(true);
 
-        $this->assertSame($user, $method->invoke($provider, 'fabien', $this->getSupportedToken()));
+        $this->assertEquals('fabien', $method->invoke($provider, 'fabien', $this->getSupportedToken())->getUserIdentifier());
     }
 
     public function testCheckAuthenticationWhenCredentialsAreEmpty()
@@ -323,14 +323,16 @@ protected function getSupportedToken()
         return $mock;
     }
 
-    protected function getProvider($user = null, $userChecker = null, $passwordHasher = null)
+    protected function getProvider($user = null, $userChecker = null, $passwordHasher = null, $userProvider = null)
     {
-        $userProvider = $this->createMock(PasswordUpgraderProvider::class);
-        if (null !== $user) {
-            $userProvider->expects($this->once())
-                         ->method('loadUserByUsername')
-                         ->willReturn($user)
-            ;
+        if (null === $userProvider) {
+            $userProvider = $this->createMock(PasswordUpgraderProvider::class);
+            if (null !== $user) {
+                $userProvider->expects($this->once())
+                             ->method('loadUserByIdentifier')
+                             ->willReturn($user)
+                ;
+            }
         }
 
         if (null === $userChecker) {
@@ -374,6 +376,11 @@ public function getUsername(): string
         return 'jane_doe';
     }
 
+    public function getUserIdentifier(): string
+    {
+        return 'jane_doe';
+    }
+
     public function eraseCredentials()
     {
     }
@@ -381,4 +388,10 @@ public function eraseCredentials()
 interface PasswordUpgraderProvider extends UserProviderInterface, PasswordUpgraderInterface
 {
     public function upgradePassword(UserInterface $user, string $newHashedPassword): void;
+    public function loadUserByIdentifier(string $identifier): UserInterface;
+}
+
+interface DaoAuthenticationProviderTest_UserProvider extends UserProviderInterface
+{
+    public function loadUserByUsername($username);
 }
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/LdapBindAuthenticationProviderTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/LdapBindAuthenticationProviderTest.php
index c47508449ffad..4507e6a9cfc72 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/LdapBindAuthenticationProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/LdapBindAuthenticationProviderTest.php
@@ -21,6 +21,7 @@
 use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
+use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 
@@ -81,10 +82,10 @@ public function testBindFailureShouldThrowAnException()
 
     public function testRetrieveUser()
     {
-        $userProvider = $this->createMock(UserProviderInterface::class);
+        $userProvider = $this->createMock(InMemoryUserProvider::class);
         $userProvider
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with('foo')
         ;
         $ldap = $this->createMock(LdapInterface::class);
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/PreAuthenticatedAuthenticationProviderTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/PreAuthenticatedAuthenticationProviderTest.php
index a0d6041327f73..15c079b8c398e 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/PreAuthenticatedAuthenticationProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/PreAuthenticatedAuthenticationProviderTest.php
@@ -18,6 +18,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\Exception\LockedException;
+use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
@@ -120,10 +121,10 @@ protected function getSupportedToken($user = false, $credentials = false)
 
     protected function getProvider($user = null, $userChecker = null)
     {
-        $userProvider = $this->createMock(UserProviderInterface::class);
+        $userProvider = $this->createMock(InMemoryUserProvider::class);
         if (null !== $user) {
             $userProvider->expects($this->once())
-                         ->method('loadUserByUsername')
+                         ->method('loadUserByIdentifier')
                          ->willReturn($user)
             ;
         }
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/UserAuthenticationProviderTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/UserAuthenticationProviderTest.php
index f006d37ed1818..92b71448d5c7d 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/UserAuthenticationProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Provider/UserAuthenticationProviderTest.php
@@ -21,7 +21,8 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationServiceException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\Exception\CredentialsExpiredException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
+use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
 
@@ -46,11 +47,11 @@ public function testAuthenticateWhenTokenIsNotSupported()
 
     public function testAuthenticateWhenUsernameIsNotFound()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
         $provider = $this->getProvider(false, false);
         $provider->expects($this->once())
                  ->method('retrieveUser')
-                 ->willThrowException(new UsernameNotFoundException())
+                 ->willThrowException(new UserNotFoundException())
         ;
 
         $provider->authenticate($this->getSupportedToken());
@@ -62,7 +63,7 @@ public function testAuthenticateWhenUsernameIsNotFoundAndHideIsTrue()
         $provider = $this->getProvider(false, true);
         $provider->expects($this->once())
                  ->method('retrieveUser')
-                 ->willThrowException(new UsernameNotFoundException())
+                 ->willThrowException(new UserNotFoundException())
         ;
 
         $provider->authenticate($this->getSupportedToken());
@@ -194,7 +195,7 @@ public function testAuthenticatePreservesOriginalToken()
         ;
 
         $originalToken = $this->createMock(TokenInterface::class);
-        $token = new SwitchUserToken($this->createMock(UserInterface::class), 'foo', 'key', [], $originalToken);
+        $token = new SwitchUserToken(new InMemoryUser('wouter', null), 'foo', 'key', [], $originalToken);
         $token->setAttributes(['foo' => 'bar']);
 
         $authToken = $provider->authenticate($token);
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/RememberMe/PersistentTokenTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/RememberMe/PersistentTokenTest.php
index 12c133f52df57..9df545a4c0c6e 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/RememberMe/PersistentTokenTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/RememberMe/PersistentTokenTest.php
@@ -12,19 +12,33 @@
 namespace Symfony\Component\Security\Core\Tests\Authentication\RememberMe;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Security\Core\Authentication\RememberMe\PersistentToken;
 
 class PersistentTokenTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     public function testConstructor()
     {
         $lastUsed = new \DateTime();
         $token = new PersistentToken('fooclass', 'fooname', 'fooseries', 'footokenvalue', $lastUsed);
 
         $this->assertEquals('fooclass', $token->getClass());
-        $this->assertEquals('fooname', $token->getUsername());
+        $this->assertEquals('fooname', $token->getUserIdentifier());
         $this->assertEquals('fooseries', $token->getSeries());
         $this->assertEquals('footokenvalue', $token->getTokenValue());
         $this->assertSame($lastUsed, $token->getLastUsed());
     }
+
+    /**
+     * @group legacy
+     */
+    public function testLegacyGetUsername()
+    {
+        $token = new PersistentToken('fooclass', 'fooname', 'fooseries', 'footokenvalue', new \DateTime());
+
+        $this->expectDeprecation('Since symfony/security-core 5.3: Method "Symfony\Component\Security\Core\Authentication\RememberMe\PersistentToken::getUsername()" is deprecated, use getUserIdentifier() instead.');
+        $this->assertEquals('fooname', $token->getUsername());
+    }
 }
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
index 98f84e1f1e077..dcf479c84e710 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/AbstractTokenTest.php
@@ -12,12 +12,19 @@
 namespace Symfony\Component\Security\Core\Tests\Authentication\Token;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Security\Core\Authentication\Token\AbstractToken;
+use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserInterface;
 
 class AbstractTokenTest extends TestCase
 {
-    public function testGetUsername()
+    use ExpectDeprecationTrait;
+
+    /**
+     * @group legacy
+     */
+    public function testLegacyGetUsername()
     {
         $token = new ConcreteToken(['ROLE_FOO']);
         $token->setUser('fabien');
@@ -26,10 +33,43 @@ public function testGetUsername()
         $token->setUser(new TestUser('fabien'));
         $this->assertEquals('fabien', $token->getUsername());
 
-        $user = $this->createMock(UserInterface::class);
-        $user->expects($this->once())->method('getUsername')->willReturn('fabien');
-        $token->setUser($user);
+        $legacyUser = new class implements UserInterface {
+            public function getUsername()
+            {
+                return 'fabien';
+            }
+
+            public function getRoles()
+            {}
+
+            public function getPassword()
+            {}
+
+            public function getSalt()
+            {}
+
+            public function eraseCredentials()
+            {}
+        };
+        $token->setUser($legacyUser);
         $this->assertEquals('fabien', $token->getUsername());
+
+        $token->setUser($legacyUser);
+        $this->assertEquals('fabien', $token->getUserIdentifier());
+    }
+
+    public function testGetUserIdentifier()
+    {
+        $token = new ConcreteToken(['ROLE_FOO']);
+        $token->setUser('fabien');
+        $this->assertEquals('fabien', $token->getUserIdentifier());
+
+        $token->setUser(new TestUser('fabien'));
+        $this->assertEquals('fabien', $token->getUserIdentifier());
+
+        $user = new InMemoryUser('fabien', null);
+        $token->setUser($user);
+        $this->assertEquals('fabien', $token->getUserIdentifier());
     }
 
     public function testEraseCredentials()
@@ -106,10 +146,8 @@ public function testSetUser($user)
 
     public function getUsers()
     {
-        $user = $this->createMock(UserInterface::class);
-
         return [
-            [$user],
+            [new InMemoryUser('foo', null)],
             [new TestUser('foo')],
             ['foo'],
         ];
@@ -210,6 +248,11 @@ public function getUsername()
         return $this->name;
     }
 
+    public function getUserIdentifier()
+    {
+        return $this->name;
+    }
+
     public function getPassword()
     {
         return '***';
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/Storage/UsageTrackingTokenStorageTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/Storage/UsageTrackingTokenStorageTest.php
index c5d2eaf543203..38806efa8a20d 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/Storage/UsageTrackingTokenStorageTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/Storage/UsageTrackingTokenStorageTest.php
@@ -16,6 +16,7 @@
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Session\SessionInterface;
+use Symfony\Component\Security\Core\Authentication\Token\NullToken;
 use Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorage;
 use Symfony\Component\Security\Core\Authentication\Token\Storage\UsageTrackingTokenStorage;
 use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
@@ -46,7 +47,7 @@ public function testGetSetToken()
         $trackingStorage = new UsageTrackingTokenStorage($tokenStorage, $sessionLocator);
 
         $this->assertNull($trackingStorage->getToken());
-        $token = $this->createMock(TokenInterface::class);
+        $token = new NullToken();
 
         $trackingStorage->setToken($token);
         $this->assertSame($token, $trackingStorage->getToken());
diff --git a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/SwitchUserTokenTest.php b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/SwitchUserTokenTest.php
index 8138f7659639b..477247e76da4f 100644
--- a/src/Symfony/Component/Security/Core/Tests/Authentication/Token/SwitchUserTokenTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Authentication/Token/SwitchUserTokenTest.php
@@ -26,7 +26,7 @@ public function testSerialize()
         $unserializedToken = unserialize(serialize($token));
 
         $this->assertInstanceOf(SwitchUserToken::class, $unserializedToken);
-        $this->assertSame('admin', $unserializedToken->getUsername());
+        $this->assertSame('admin', $unserializedToken->getUserIdentifier());
         $this->assertSame('bar', $unserializedToken->getCredentials());
         $this->assertSame('provider-key', $unserializedToken->getFirewallName());
         $this->assertEquals(['ROLE_USER'], $unserializedToken->getRoleNames());
@@ -35,7 +35,7 @@ public function testSerialize()
         $unserializedOriginalToken = $unserializedToken->getOriginalToken();
 
         $this->assertInstanceOf(UsernamePasswordToken::class, $unserializedOriginalToken);
-        $this->assertSame('user', $unserializedOriginalToken->getUsername());
+        $this->assertSame('user', $unserializedOriginalToken->getUserIdentifier());
         $this->assertSame('foo', $unserializedOriginalToken->getCredentials());
         $this->assertSame('provider-key', $unserializedOriginalToken->getFirewallName());
         $this->assertEquals(['ROLE_ADMIN', 'ROLE_ALLOWED_TO_SWITCH'], $unserializedOriginalToken->getRoleNames());
@@ -49,6 +49,11 @@ public function getUsername()
                 return 'impersonated';
             }
 
+            public function getUserIdentifier()
+            {
+                return 'impersonated';
+            }
+
             public function getPassword()
             {
                 return null;
@@ -92,7 +97,7 @@ public function testUnserializeOldToken()
 
         self::assertInstanceOf(SwitchUserToken::class, $token);
         self::assertInstanceOf(UsernamePasswordToken::class, $token->getOriginalToken());
-        self::assertSame('john', $token->getUsername());
+        self::assertSame('john', $token->getUserIdentifier());
         self::assertSame(['foo' => 'bar'], $token->getCredentials());
         self::assertSame('main', $token->getFirewallName());
         self::assertEquals(['ROLE_USER'], $token->getRoleNames());
diff --git a/src/Symfony/Component/Security/Core/Tests/Encoder/EncoderFactoryTest.php b/src/Symfony/Component/Security/Core/Tests/Encoder/EncoderFactoryTest.php
index 7b79986b826a6..3744e05bc0efa 100644
--- a/src/Symfony/Component/Security/Core/Tests/Encoder/EncoderFactoryTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/Encoder/EncoderFactoryTest.php
@@ -213,6 +213,10 @@ public function getUsername(): string
     {
     }
 
+    public function getUserIdentifier(): string
+    {
+    }
+
     public function eraseCredentials()
     {
     }
diff --git a/src/Symfony/Component/Security/Core/Tests/Exception/UserNotFoundExceptionTest.php b/src/Symfony/Component/Security/Core/Tests/Exception/UserNotFoundExceptionTest.php
new file mode 100644
index 0000000000000..559e62acd97d0
--- /dev/null
+++ b/src/Symfony/Component/Security/Core/Tests/Exception/UserNotFoundExceptionTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Security\Core\Tests\Exception;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
+use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+
+class UserNotFoundExceptionTest extends TestCase
+{
+    public function testGetMessageData()
+    {
+        $exception = new UserNotFoundException('Username could not be found.');
+        $this->assertEquals(['{{ username }}' => null, '{{ user_identifier }}' => null], $exception->getMessageData());
+        $exception->setUserIdentifier('username');
+        $this->assertEquals(['{{ username }}' => 'username', '{{ user_identifier }}' => 'username'], $exception->getMessageData());
+    }
+
+    /**
+     * @group legacy
+     */
+    public function testUsernameNotFoundException()
+    {
+        $exception = new UsernameNotFoundException();
+        $this->assertInstanceOf(UserNotFoundException::class, $exception);
+
+        $exception->setUsername('username');
+        $this->assertEquals('username', $exception->getUserIdentifier());
+    }
+}
diff --git a/src/Symfony/Component/Security/Core/Tests/Exception/UsernameNotFoundExceptionTest.php b/src/Symfony/Component/Security/Core/Tests/Exception/UsernameNotFoundExceptionTest.php
deleted file mode 100644
index 8e256aac2ea11..0000000000000
--- a/src/Symfony/Component/Security/Core/Tests/Exception/UsernameNotFoundExceptionTest.php
+++ /dev/null
@@ -1,26 +0,0 @@
-<?php
-
-/*
- * This file is part of the Symfony package.
- *
- * (c) Fabien Potencier <fabien@symfony.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-namespace Symfony\Component\Security\Core\Tests\Exception;
-
-use PHPUnit\Framework\TestCase;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
-
-class UsernameNotFoundExceptionTest extends TestCase
-{
-    public function testGetMessageData()
-    {
-        $exception = new UsernameNotFoundException('Username could not be found.');
-        $this->assertEquals(['{{ username }}' => null], $exception->getMessageData());
-        $exception->setUsername('username');
-        $this->assertEquals(['{{ username }}' => 'username'], $exception->getMessageData());
-    }
-}
diff --git a/src/Symfony/Component/Security/Core/Tests/User/ChainUserProviderTest.php b/src/Symfony/Component/Security/Core/Tests/User/ChainUserProviderTest.php
index 74d0cc138ce6f..5a4770064fc2e 100644
--- a/src/Symfony/Component/Security/Core/Tests/User/ChainUserProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/User/ChainUserProviderTest.php
@@ -13,9 +13,10 @@
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\ChainUserProvider;
 use Symfony\Component\Security\Core\User\InMemoryUser;
+use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -25,59 +26,59 @@ class ChainUserProviderTest extends TestCase
 {
     public function testLoadUserByUsername()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with($this->equalTo('foo'))
-            ->willThrowException(new UsernameNotFoundException('not found'))
+            ->willThrowException(new UserNotFoundException('not found'))
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with($this->equalTo('foo'))
             ->willReturn($account = $this->createMock(UserInterface::class))
         ;
 
         $provider = new ChainUserProvider([$provider1, $provider2]);
-        $this->assertSame($account, $provider->loadUserByUsername('foo'));
+        $this->assertSame($account, $provider->loadUserByIdentifier('foo'));
     }
 
-    public function testLoadUserByUsernameThrowsUsernameNotFoundException()
+    public function testLoadUserByUsernameThrowsUserNotFoundException()
     {
-        $this->expectException(UsernameNotFoundException::class);
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $this->expectException(UserNotFoundException::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with($this->equalTo('foo'))
-            ->willThrowException(new UsernameNotFoundException('not found'))
+            ->willThrowException(new UserNotFoundException('not found'))
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
-            ->method('loadUserByUsername')
+            ->method('loadUserByIdentifier')
             ->with($this->equalTo('foo'))
-            ->willThrowException(new UsernameNotFoundException('not found'))
+            ->willThrowException(new UserNotFoundException('not found'))
         ;
 
         $provider = new ChainUserProvider([$provider1, $provider2]);
-        $provider->loadUserByUsername('foo');
+        $provider->loadUserByIdentifier('foo');
     }
 
     public function testRefreshUser()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
             ->willReturn(false)
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
@@ -90,7 +91,7 @@ public function testRefreshUser()
             ->willThrowException(new UnsupportedUserException('unsupported'))
         ;
 
-        $provider3 = $this->createMock(UserProviderInterface::class);
+        $provider3 = $this->createMock(InMemoryUserProvider::class);
         $provider3
             ->expects($this->once())
             ->method('supportsClass')
@@ -109,7 +110,7 @@ public function testRefreshUser()
 
     public function testRefreshUserAgain()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
@@ -119,10 +120,10 @@ public function testRefreshUserAgain()
         $provider1
             ->expects($this->once())
             ->method('refreshUser')
-            ->willThrowException(new UsernameNotFoundException('not found'))
+            ->willThrowException(new UserNotFoundException('not found'))
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
@@ -142,7 +143,7 @@ public function testRefreshUserAgain()
     public function testRefreshUserThrowsUnsupportedUserException()
     {
         $this->expectException(UnsupportedUserException::class);
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
@@ -155,7 +156,7 @@ public function testRefreshUserThrowsUnsupportedUserException()
             ->willThrowException(new UnsupportedUserException('unsupported'))
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
@@ -174,7 +175,7 @@ public function testRefreshUserThrowsUnsupportedUserException()
 
     public function testSupportsClass()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
@@ -182,7 +183,7 @@ public function testSupportsClass()
             ->willReturn(false)
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
@@ -196,7 +197,7 @@ public function testSupportsClass()
 
     public function testSupportsClassWhenNotSupported()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
@@ -204,7 +205,7 @@ public function testSupportsClassWhenNotSupported()
             ->willReturn(false)
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
@@ -218,7 +219,7 @@ public function testSupportsClassWhenNotSupported()
 
     public function testAcceptsTraversable()
     {
-        $provider1 = $this->createMock(UserProviderInterface::class);
+        $provider1 = $this->createMock(InMemoryUserProvider::class);
         $provider1
             ->expects($this->once())
             ->method('supportsClass')
@@ -231,7 +232,7 @@ public function testAcceptsTraversable()
             ->willThrowException(new UnsupportedUserException('unsupported'))
         ;
 
-        $provider2 = $this->createMock(UserProviderInterface::class);
+        $provider2 = $this->createMock(InMemoryUserProvider::class);
         $provider2
             ->expects($this->once())
             ->method('supportsClass')
diff --git a/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserProviderTest.php b/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserProviderTest.php
index f9d27c8f844a3..d4d4964c7be59 100644
--- a/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserProviderTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserProviderTest.php
@@ -13,7 +13,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\User;
@@ -26,7 +26,7 @@ public function testConstructor()
     {
         $provider = $this->createProvider();
 
-        $user = $provider->loadUserByUsername('fabien');
+        $user = $provider->loadUserByIdentifier('fabien');
         $this->assertEquals('foo', $user->getPassword());
         $this->assertEquals(['ROLE_USER'], $user->getRoles());
         $this->assertFalse($user->isEnabled());
@@ -76,7 +76,7 @@ public function testCreateUser()
         $provider = new InMemoryUserProvider();
         $provider->createUser(new InMemoryUser('fabien', 'foo'));
 
-        $user = $provider->loadUserByUsername('fabien');
+        $user = $provider->loadUserByIdentifier('fabien');
         $this->assertEquals('foo', $user->getPassword());
     }
 
@@ -90,8 +90,8 @@ public function testCreateUserAlreadyExist()
 
     public function testLoadUserByUsernameDoesNotExist()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
         $provider = new InMemoryUserProvider();
-        $provider->loadUserByUsername('fabien');
+        $provider->loadUserByIdentifier('fabien');
     }
 }
diff --git a/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserTest.php b/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserTest.php
index 885d1f73c0690..a5496ef325b85 100644
--- a/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/User/InMemoryUserTest.php
@@ -12,12 +12,15 @@
 namespace Symfony\Component\Security\Core\Tests\User;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Security\Core\User\EquatableInterface;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserInterface;
 
 class InMemoryUserTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     public function testConstructorException()
     {
         $this->expectException(\InvalidArgumentException::class);
@@ -39,12 +42,23 @@ public function testGetPassword()
         $this->assertEquals('superpass', $user->getPassword());
     }
 
+    /**
+     * @group legacy
+     */
     public function testGetUsername()
     {
         $user = new InMemoryUser('fabien', 'superpass');
+
+        $this->expectDeprecation('Since symfony/security-core 5.3: Method "Symfony\Component\Security\Core\User\User::getUsername()" is deprecated, use getUserIdentifier() instead.');
         $this->assertEquals('fabien', $user->getUsername());
     }
 
+    public function testGetUserIdentifier()
+    {
+        $user = new InMemoryUser('fabien', 'superpass');
+        $this->assertEquals('fabien', $user->getUserIdentifier());
+    }
+
     public function testGetSalt()
     {
         $user = new InMemoryUser('fabien', 'superpass');
diff --git a/src/Symfony/Component/Security/Core/Tests/User/UserTest.php b/src/Symfony/Component/Security/Core/Tests/User/UserTest.php
index 143479de7940d..81b8705d1041f 100644
--- a/src/Symfony/Component/Security/Core/Tests/User/UserTest.php
+++ b/src/Symfony/Component/Security/Core/Tests/User/UserTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Security\Core\Tests\User;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Security\Core\User\EquatableInterface;
 use Symfony\Component\Security\Core\User\User;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -21,6 +22,8 @@
  */
 class UserTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     public function testConstructorException()
     {
         $this->expectException(\InvalidArgumentException::class);
@@ -42,12 +45,23 @@ public function testGetPassword()
         $this->assertEquals('superpass', $user->getPassword());
     }
 
+    /**
+     * @group legacy
+     */
     public function testGetUsername()
     {
         $user = new User('fabien', 'superpass');
+
+        $this->expectDeprecation('Since symfony/security-core 5.3: Method "Symfony\Component\Security\Core\User\User::getUsername()" is deprecated, use getUserIdentifier() instead.');
         $this->assertEquals('fabien', $user->getUsername());
     }
 
+    public function testGetUserIdentifier()
+    {
+        $user = new User('fabien', 'superpass');
+        $this->assertEquals('fabien', $user->getUserIdentifier());
+    }
+
     public function testGetSalt()
     {
         $user = new User('fabien', 'superpass');
diff --git a/src/Symfony/Component/Security/Core/User/ChainUserProvider.php b/src/Symfony/Component/Security/Core/User/ChainUserProvider.php
index fedcdb6a53c10..35207d62cc924 100644
--- a/src/Symfony/Component/Security/Core/User/ChainUserProvider.php
+++ b/src/Symfony/Component/Security/Core/User/ChainUserProvider.php
@@ -12,7 +12,7 @@
 namespace Symfony\Component\Security\Core\User;
 
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 
 /**
  * Chain User Provider.
@@ -50,17 +50,31 @@ public function getProviders()
      * {@inheritdoc}
      */
     public function loadUserByUsername(string $username)
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use loadUserByIdentifier() instead.', __METHOD__);
+
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $userIdentifier): UserInterface
     {
         foreach ($this->providers as $provider) {
             try {
-                return $provider->loadUserByUsername($username);
-            } catch (UsernameNotFoundException $e) {
+                // @deprecated since 5.3, change to $provider->loadUserByIdentifier() in 6.0
+                if (!method_exists($provider, 'loadUserByIdentifier')) {
+                    trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', \get_debug_type($provider));
+
+                    return $provider->loadUserByUsername($userIdentifier);
+                }
+
+                return $provider->loadUserByIdentifier($userIdentifier);
+            } catch (UserNotFoundException $e) {
                 // try next one
             }
         }
 
-        $ex = new UsernameNotFoundException(sprintf('There is no user with name "%s".', $username));
-        $ex->setUsername($username);
+        $ex = new UserNotFoundException(sprintf('There is no user with identifier "%s".', $userIdentifier));
+        $ex->setUserIdentifier($userIdentifier);
         throw $ex;
     }
 
@@ -80,15 +94,17 @@ public function refreshUser(UserInterface $user)
                 return $provider->refreshUser($user);
             } catch (UnsupportedUserException $e) {
                 // try next one
-            } catch (UsernameNotFoundException $e) {
+            } catch (UserNotFoundException $e) {
                 $supportedUserFound = true;
                 // try next one
             }
         }
 
         if ($supportedUserFound) {
-            $e = new UsernameNotFoundException(sprintf('There is no user with name "%s".', $user->getUsername()));
-            $e->setUsername($user->getUsername());
+            // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+            $username = method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername();
+            $e = new UserNotFoundException(sprintf('There is no user with name "%s".', $username));
+            $e->setUserIdentifier($username);
             throw $e;
         } else {
             throw new UnsupportedUserException(sprintf('There is no user provider for user "%s". Shouldn\'t the "supportsClass()" method of your user provider return true for this classname?', get_debug_type($user)));
diff --git a/src/Symfony/Component/Security/Core/User/InMemoryUserProvider.php b/src/Symfony/Component/Security/Core/User/InMemoryUserProvider.php
index c79f96e2542ee..2e9ea5a27f675 100644
--- a/src/Symfony/Component/Security/Core/User/InMemoryUserProvider.php
+++ b/src/Symfony/Component/Security/Core/User/InMemoryUserProvider.php
@@ -12,7 +12,7 @@
 namespace Symfony\Component\Security\Core\User;
 
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 
 /**
  * InMemoryUserProvider is a simple non persistent user provider.
@@ -51,11 +51,13 @@ public function __construct(array $users = [])
      */
     public function createUser(UserInterface $user)
     {
-        if (isset($this->users[strtolower($user->getUsername())])) {
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        $userIdentifier = strtolower(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername());
+        if (isset($this->users[$userIdentifier])) {
             throw new \LogicException('Another user with the same username already exists.');
         }
 
-        $this->users[strtolower($user->getUsername())] = $user;
+        $this->users[$userIdentifier] = $user;
     }
 
     /**
@@ -63,9 +65,17 @@ public function createUser(UserInterface $user)
      */
     public function loadUserByUsername(string $username)
     {
-        $user = $this->getUser($username);
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use loadUserByIdentifier() instead.', __METHOD__);
 
-        return new InMemoryUser($user->getUsername(), $user->getPassword(), $user->getRoles(), $user->isEnabled());
+        return $this->loadUserByIdentifier($username);
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+        $user = $this->getUser($identifier);
+
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        return new InMemoryUser(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(), $user->getPassword(), $user->getRoles(), $user->isEnabled());
     }
 
     /**
@@ -77,7 +87,9 @@ public function refreshUser(UserInterface $user)
             throw new UnsupportedUserException(sprintf('Instances of "%s" are not supported.', get_debug_type($user)));
         }
 
-        $storedUser = $this->getUser($user->getUsername());
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        $storedUser = $this->getUser(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername());
+        $userIdentifier = method_exists($storedUser, 'getUserIdentifier') ? $storedUser->getUserIdentifier() : $storedUser->getUsername();
 
         // @deprecated since Symfony 5.3
         if (User::class === \get_class($user)) {
@@ -91,10 +103,10 @@ public function refreshUser(UserInterface $user)
                 $accountNonLocked = $storedUser->isAccountNonLocked();
             }
 
-            return new User($storedUser->getUsername(), $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled(), $accountNonExpired, $credentialsNonExpired, $accountNonLocked);
+            return new User($userIdentifier, $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled(), $accountNonExpired, $credentialsNonExpired, $accountNonLocked);
         }
 
-        return new InMemoryUser($storedUser->getUsername(), $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled());
+        return new InMemoryUser($userIdentifier, $storedUser->getPassword(), $storedUser->getRoles(), $storedUser->isEnabled());
     }
 
     /**
@@ -113,13 +125,13 @@ public function supportsClass(string $class)
     /**
      * Returns the user by given username.
      *
-     * @throws UsernameNotFoundException if user whose given username does not exist
+     * @throws UserNotFoundException if user whose given username does not exist
      */
     private function getUser(string $username)/*: InMemoryUser */
     {
         if (!isset($this->users[strtolower($username)])) {
-            $ex = new UsernameNotFoundException(sprintf('Username "%s" does not exist.', $username));
-            $ex->setUsername($username);
+            $ex = new UserNotFoundException(sprintf('Username "%s" does not exist.', $username));
+            $ex->setUserIdentifier($username);
 
             throw $ex;
         }
diff --git a/src/Symfony/Component/Security/Core/User/MissingUserProvider.php b/src/Symfony/Component/Security/Core/User/MissingUserProvider.php
index 605aad6dfc74b..02df05169e9b8 100644
--- a/src/Symfony/Component/Security/Core/User/MissingUserProvider.php
+++ b/src/Symfony/Component/Security/Core/User/MissingUserProvider.php
@@ -37,6 +37,11 @@ public function loadUserByUsername(string $username): UserInterface
         throw new \BadMethodCallException();
     }
 
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+        throw new \BadMethodCallException();
+    }
+
     /**
      * {@inheritdoc}
      */
diff --git a/src/Symfony/Component/Security/Core/User/User.php b/src/Symfony/Component/Security/Core/User/User.php
index 045f03bff2554..d583e5a8cbd4f 100644
--- a/src/Symfony/Component/Security/Core/User/User.php
+++ b/src/Symfony/Component/Security/Core/User/User.php
@@ -53,7 +53,7 @@ public function __construct(?string $username, ?string $password, array $roles =
 
     public function __toString(): string
     {
-        return $this->getUsername();
+        return $this->getUserIdentifier();
     }
 
     /**
@@ -84,6 +84,16 @@ public function getSalt(): ?string
      * {@inheritdoc}
      */
     public function getUsername(): string
+    {
+        trigger_deprecation('symfony/security-core', '5.3', 'Method "%s()" is deprecated, use getUserIdentifier() instead.', __METHOD__);
+
+        return $this->username;
+    }
+
+    /**
+     * Returns the identifier for this user (e.g. its username or e-mailaddress).
+     */
+    public function getUserIdentifier(): string
     {
         return $this->username;
     }
@@ -184,7 +194,7 @@ public function isEqualTo(UserInterface $user): bool
             return false;
         }
 
-        if ($this->getUsername() !== $user->getUsername()) {
+        if ($this->getUserIdentifier() !== $user->getUserIdentifier()) {
             return false;
         }
 
diff --git a/src/Symfony/Component/Security/Core/User/UserInterface.php b/src/Symfony/Component/Security/Core/User/UserInterface.php
index 47661de0b7317..6448ab519248d 100644
--- a/src/Symfony/Component/Security/Core/User/UserInterface.php
+++ b/src/Symfony/Component/Security/Core/User/UserInterface.php
@@ -26,6 +26,8 @@
  *
  * @see UserProviderInterface
  *
+ * @method string getUserIdentifier() returns the identifier for this user (e.g. its username or e-mailaddress)
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 interface UserInterface
@@ -69,13 +71,6 @@ public function getPassword();
      */
     public function getSalt();
 
-    /**
-     * Returns the username used to authenticate the user.
-     *
-     * @return string The username
-     */
-    public function getUsername();
-
     /**
      * Removes sensitive data from the user.
      *
diff --git a/src/Symfony/Component/Security/Core/User/UserProviderInterface.php b/src/Symfony/Component/Security/Core/User/UserProviderInterface.php
index 708a97f4be404..5ab67836c730f 100644
--- a/src/Symfony/Component/Security/Core/User/UserProviderInterface.php
+++ b/src/Symfony/Component/Security/Core/User/UserProviderInterface.php
@@ -12,16 +12,16 @@
 namespace Symfony\Component\Security\Core\User;
 
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 
 /**
  * Represents a class that loads UserInterface objects from some source for the authentication system.
  *
- * In a typical authentication configuration, a username (i.e. some unique
- * user identifier) credential enters the system (via form login, or any
- * method). The user provider that is configured with that authentication
- * method is asked to load the UserInterface object for the given username
- * (via loadUserByUsername) so that the rest of the process can continue.
+ * In a typical authentication configuration, a user identifier (e.g. a
+ * username or e-mailaddress) credential enters the system (via form login, or
+ * any method). The user provider that is configured with that authentication
+ * method is asked to load the UserInterface object for the given identifier (via
+ * loadUserByIdentifier) so that the rest of the process can continue.
  *
  * Internally, a user provider can load users from any source (databases,
  * configuration, web service). This is totally independent of how the authentication
@@ -29,22 +29,13 @@
  *
  * @see UserInterface
  *
+ * @method UserInterface loadUserByIdentifier(string $identifier) loads the user for the given user identifier (e.g. username or email).
+ *                                                                This method must throw UserNotFoundException if the user is not found.
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 interface UserProviderInterface
 {
-    /**
-     * Loads the user for the given username.
-     *
-     * This method must throw UsernameNotFoundException if the user is not
-     * found.
-     *
-     * @return UserInterface
-     *
-     * @throws UsernameNotFoundException if the user is not found
-     */
-    public function loadUserByUsername(string $username);
-
     /**
      * Refreshes the user.
      *
@@ -55,8 +46,8 @@ public function loadUserByUsername(string $username);
      *
      * @return UserInterface
      *
-     * @throws UnsupportedUserException  if the user is not supported
-     * @throws UsernameNotFoundException if the user is not found
+     * @throws UnsupportedUserException if the user is not supported
+     * @throws UserNotFoundException    if the user is not found
      */
     public function refreshUser(UserInterface $user);
 
diff --git a/src/Symfony/Component/Security/Guard/Authenticator/GuardBridgeAuthenticator.php b/src/Symfony/Component/Security/Guard/Authenticator/GuardBridgeAuthenticator.php
index 8e678227806fa..b3384111008df 100644
--- a/src/Symfony/Component/Security/Guard/Authenticator/GuardBridgeAuthenticator.php
+++ b/src/Symfony/Component/Security/Guard/Authenticator/GuardBridgeAuthenticator.php
@@ -15,7 +15,7 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserInterface;
@@ -98,7 +98,7 @@ private function getUser($credentials): UserInterface
         $user = $this->guard->getUser($credentials, $this->userProvider);
 
         if (null === $user) {
-            throw new UsernameNotFoundException(sprintf('Null returned from "%s::getUser()".', get_debug_type($this->guard)));
+            throw new UserNotFoundException(sprintf('Null returned from "%s::getUser()".', get_debug_type($this->guard)));
         }
 
         if (!$user instanceof UserInterface) {
diff --git a/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php b/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php
index 36c1be3115855..870ba387822a3 100644
--- a/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php
+++ b/src/Symfony/Component/Security/Guard/AuthenticatorInterface.php
@@ -70,7 +70,7 @@ public function getCredentials(Request $request);
      * The *credentials* are the return value from getCredentials()
      *
      * You may throw an AuthenticationException if you wish. If you return
-     * null, then a UsernameNotFoundException is thrown for you.
+     * null, then a UserNotFoundException is thrown for you.
      *
      * @param mixed $credentials
      *
diff --git a/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php b/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
index 71d663e3dcf85..880d275f68e65 100644
--- a/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
@@ -18,7 +18,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\AuthenticationExpiredException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\PasswordAuthenticatedUserInterface;
 use Symfony\Component\Security\Core\User\PasswordUpgraderInterface;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
@@ -112,8 +112,9 @@ private function authenticateViaGuard(AuthenticatorInterface $guardAuthenticator
         $user = $guardAuthenticator->getUser($token->getCredentials(), $this->userProvider);
 
         if (null === $user) {
-            $e = new UsernameNotFoundException(sprintf('Null returned from "%s::getUser()".', get_debug_type($guardAuthenticator)));
-            $e->setUsername($token->getUsername());
+            $e = new UserNotFoundException(sprintf('Null returned from "%s::getUser()".', get_debug_type($guardAuthenticator)));
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            $e->setUserIdentifier(method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername());
 
             throw $e;
         }
diff --git a/src/Symfony/Component/Security/Guard/Tests/Authenticator/GuardBridgeAuthenticatorTest.php b/src/Symfony/Component/Security/Guard/Tests/Authenticator/GuardBridgeAuthenticatorTest.php
index 678e1f17ceb7c..24a4d71c509d5 100644
--- a/src/Symfony/Component/Security/Guard/Tests/Authenticator/GuardBridgeAuthenticatorTest.php
+++ b/src/Symfony/Component/Security/Guard/Tests/Authenticator/GuardBridgeAuthenticatorTest.php
@@ -15,7 +15,7 @@
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Guard\Authenticator\GuardBridgeAuthenticator;
@@ -97,7 +97,7 @@ public function testAuthenticate()
 
     public function testAuthenticateNoUser()
     {
-        $this->expectException(UsernameNotFoundException::class);
+        $this->expectException(UserNotFoundException::class);
 
         $request = new Request();
 
diff --git a/src/Symfony/Component/Security/Http/Authentication/AuthenticatorManager.php b/src/Symfony/Component/Security/Http/Authentication/AuthenticatorManager.php
index 501a9e2fb1a3c..b5e609c74196b 100644
--- a/src/Symfony/Component/Security/Http/Authentication/AuthenticatorManager.php
+++ b/src/Symfony/Component/Security/Http/Authentication/AuthenticatorManager.php
@@ -70,7 +70,8 @@ public function __construct(iterable $authenticators, TokenStorageInterface $tok
     public function authenticateUser(UserInterface $user, AuthenticatorInterface $authenticator, Request $request, array $badges = []): ?Response
     {
         // create an authenticated token for the User
-        $token = $authenticator->createAuthenticatedToken($passport = new SelfValidatingPassport(new UserBadge($user->getUsername(), function () use ($user) { return $user; }), $badges), $this->firewallName);
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        $token = $authenticator->createAuthenticatedToken($passport = new SelfValidatingPassport(new UserBadge(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(), function () use ($user) { return $user; }), $badges), $this->firewallName);
 
         // announce the authenticated token
         $token = $this->eventDispatcher->dispatch(new AuthenticationTokenCreatedEvent($token))->getAuthenticatedToken();
@@ -215,6 +216,11 @@ private function executeAuthenticator(AuthenticatorInterface $authenticator, Req
 
     private function handleAuthenticationSuccess(TokenInterface $authenticatedToken, PassportInterface $passport, Request $request, AuthenticatorInterface $authenticator): ?Response
     {
+        // @deprecated since 5.3
+        if (!method_exists($authenticatedToken->getUser(), 'getUserIdentifier')) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "getUserIdentifier(): string" in user class "%s" is deprecated. This method will replace "getUsername()" in Symfony 6.0.', get_debug_type($authenticatedToken->getUser()));
+        }
+
         $this->tokenStorage->setToken($authenticatedToken);
 
         $response = $authenticator->onAuthenticationSuccess($request, $authenticatedToken, $this->firewallName);
diff --git a/src/Symfony/Component/Security/Http/Authenticator/AbstractPreAuthenticatedAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/AbstractPreAuthenticatedAuthenticator.php
index 8ccd356ca1d09..cb8470f4e856c 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/AbstractPreAuthenticatedAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/AbstractPreAuthenticatedAuthenticator.php
@@ -87,9 +87,18 @@ public function supports(Request $request): ?bool
 
     public function authenticate(Request $request): PassportInterface
     {
-        return new SelfValidatingPassport(new UserBadge($request->attributes->get('_pre_authenticated_username'), function ($username) {
-            return $this->userProvider->loadUserByUsername($username);
-        }), [new PreAuthenticatedUserBadge()]);
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        $method = 'loadUserByIdentifier';
+        if (!method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $method = 'loadUserByUsername';
+        }
+
+        return new SelfValidatingPassport(
+            new UserBadge($request->attributes->get('_pre_authenticated_username'), [$this->userProvider, $method]),
+            [new PreAuthenticatedUserBadge()]
+        );
     }
 
     public function createAuthenticatedToken(PassportInterface $passport, string $firewallName): TokenInterface
diff --git a/src/Symfony/Component/Security/Http/Authenticator/AuthenticatorInterface.php b/src/Symfony/Component/Security/Http/Authenticator/AuthenticatorInterface.php
index 0d7e9cb7bb6f4..d6ec043d15cfc 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/AuthenticatorInterface.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/AuthenticatorInterface.php
@@ -46,7 +46,7 @@ public function supports(Request $request): ?bool;
      * presented password and the CSRF token value.
      *
      * You may throw any AuthenticationException in this method in case of error (e.g.
-     * a UsernameNotFoundException when the user cannot be found).
+     * a UserNotFoundException when the user cannot be found).
      *
      * @throws AuthenticationException
      */
diff --git a/src/Symfony/Component/Security/Http/Authenticator/FormLoginAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/FormLoginAuthenticator.php
index 234b2dc0a3d5a..965cbfa4ec99b 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/FormLoginAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/FormLoginAuthenticator.php
@@ -84,14 +84,19 @@ public function authenticate(Request $request): PassportInterface
     {
         $credentials = $this->getCredentials($request);
 
-        $passport = new Passport(new UserBadge($credentials['username'], function ($username) {
-            $user = $this->userProvider->loadUserByUsername($username);
-            if (!$user instanceof UserInterface) {
-                throw new AuthenticationServiceException('The user provider must return a UserInterface object.');
-            }
-
-            return $user;
-        }), new PasswordCredentials($credentials['password']), [new RememberMeBadge()]);
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        $method = 'loadUserByIdentifier';
+        if (!method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $method = 'loadUserByUsername';
+        }
+
+        $passport = new Passport(
+            new UserBadge($credentials['username'], [$this->userProvider, $method]),
+            new PasswordCredentials($credentials['password']),
+            [new RememberMeBadge()]
+        );
         if ($this->options['enable_csrf']) {
             $passport->addBadge(new CsrfTokenBadge($this->options['csrf_token_id'], $credentials['csrf_token']));
         }
diff --git a/src/Symfony/Component/Security/Http/Authenticator/HttpBasicAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/HttpBasicAuthenticator.php
index 179e1fdd1a94c..c184de8477747 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/HttpBasicAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/HttpBasicAuthenticator.php
@@ -67,14 +67,18 @@ public function authenticate(Request $request): PassportInterface
         $username = $request->headers->get('PHP_AUTH_USER');
         $password = $request->headers->get('PHP_AUTH_PW', '');
 
-        $passport = new Passport(new UserBadge($username, function ($username) {
-            $user = $this->userProvider->loadUserByUsername($username);
-            if (!$user instanceof UserInterface) {
-                throw new AuthenticationServiceException('The user provider must return a UserInterface object.');
-            }
-
-            return $user;
-        }), new PasswordCredentials($password));
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        $method = 'loadUserByIdentifier';
+        if (!method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $method = 'loadUserByUsername';
+        }
+
+        $passport = new Passport(
+            new UserBadge($username, [$this->userProvider, $method]),
+            new PasswordCredentials($password)
+        );
         if ($this->userProvider instanceof PasswordUpgraderInterface) {
             $passport->addBadge(new PasswordUpgradeBadge($password, $this->userProvider));
         }
diff --git a/src/Symfony/Component/Security/Http/Authenticator/JsonLoginAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/JsonLoginAuthenticator.php
index 69ef4d28f5969..8e5d3afd5c920 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/JsonLoginAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/JsonLoginAuthenticator.php
@@ -94,14 +94,18 @@ public function authenticate(Request $request): PassportInterface
             throw $e;
         }
 
-        $passport = new Passport(new UserBadge($credentials['username'], function ($username) {
-            $user = $this->userProvider->loadUserByUsername($username);
-            if (!$user instanceof UserInterface) {
-                throw new AuthenticationServiceException('The user provider must return a UserInterface object.');
-            }
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        $method = 'loadUserByIdentifier';
+        if (!method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $method = 'loadUserByUsername';
+        }
 
-            return $user;
-        }), new PasswordCredentials($credentials['password']));
+        $passport = new Passport(
+            new UserBadge($credentials['username'], [$this->userProvider, $method]),
+            new PasswordCredentials($credentials['password'])
+        );
         if ($this->userProvider instanceof PasswordUpgraderInterface) {
             $passport->addBadge(new PasswordUpgradeBadge($credentials['password'], $this->userProvider));
         }
diff --git a/src/Symfony/Component/Security/Http/Authenticator/Passport/Badge/UserBadge.php b/src/Symfony/Component/Security/Http/Authenticator/Passport/Badge/UserBadge.php
index a58f86cd13e4c..41e14141f71f8 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/Passport/Badge/UserBadge.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/Passport/Badge/UserBadge.php
@@ -11,7 +11,7 @@
 
 namespace Symfony\Component\Security\Http\Authenticator\Passport\Badge;
 
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\AuthenticationServiceException;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Http\EventListener\UserProviderListener;
 
@@ -40,7 +40,7 @@ class UserBadge implements BadgeInterface
      * based on email *and* company name). This string can be used for e.g. login throttling.
      *
      * Optionally, you may pass a user loader. This callable receives the $userIdentifier
-     * as argument and must return a UserInterface object (otherwise a UsernameNotFoundException
+     * as argument and must return a UserInterface object (otherwise an AuthenticationServiceException
      * is thrown). If this is not set, the default user provider will be used with
      * $userIdentifier as username.
      */
@@ -64,7 +64,7 @@ public function getUser(): UserInterface
 
             $this->user = ($this->userLoader)($this->userIdentifier);
             if (!$this->user instanceof UserInterface) {
-                throw new UsernameNotFoundException();
+                throw new AuthenticationServiceException(sprintf('The user provider must return a UserInterface object, "%s" given.', \get_debug_type($this->user)));
             }
         }
 
diff --git a/src/Symfony/Component/Security/Http/Authenticator/RememberMeAuthenticator.php b/src/Symfony/Component/Security/Http/Authenticator/RememberMeAuthenticator.php
index bc7ecc3996407..b663a275062dc 100644
--- a/src/Symfony/Component/Security/Http/Authenticator/RememberMeAuthenticator.php
+++ b/src/Symfony/Component/Security/Http/Authenticator/RememberMeAuthenticator.php
@@ -82,7 +82,8 @@ public function authenticate(Request $request): PassportInterface
             throw new \LogicException('No remember me token is set.');
         }
 
-        return new SelfValidatingPassport(new UserBadge($token->getUsername(), [$token, 'getUser']));
+        // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+        return new SelfValidatingPassport(new UserBadge(method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername(), [$token, 'getUser']));
     }
 
     public function createAuthenticatedToken(PassportInterface $passport, string $firewallName): TokenInterface
diff --git a/src/Symfony/Component/Security/Http/EventListener/UserProviderListener.php b/src/Symfony/Component/Security/Http/EventListener/UserProviderListener.php
index 715ae675c0bef..21fbf97daafad 100644
--- a/src/Symfony/Component/Security/Http/EventListener/UserProviderListener.php
+++ b/src/Symfony/Component/Security/Http/EventListener/UserProviderListener.php
@@ -46,6 +46,13 @@ public function checkPassport(CheckPassportEvent $event): void
             return;
         }
 
-        $badge->setUserLoader([$this->userProvider, 'loadUserByUsername']);
+        // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+        if (method_exists($this->userProvider, 'loadUserByIdentifier')) {
+            $badge->setUserLoader([$this->userProvider, 'loadUserByIdentifier']);
+        } else {
+            trigger_deprecation('symfony/security-http', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+            $badge->setUserLoader([$this->userProvider, 'loadUserByUsername']);
+        }
     }
 }
diff --git a/src/Symfony/Component/Security/Http/Firewall/AbstractAuthenticationListener.php b/src/Symfony/Component/Security/Http/Firewall/AbstractAuthenticationListener.php
index 1e4405e629158..616a4efdd7b9b 100644
--- a/src/Symfony/Component/Security/Http/Firewall/AbstractAuthenticationListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/AbstractAuthenticationListener.php
@@ -195,7 +195,8 @@ private function onFailure(Request $request, AuthenticationException $failed): R
     private function onSuccess(Request $request, TokenInterface $token): Response
     {
         if (null !== $this->logger) {
-            $this->logger->info('User has been authenticated successfully.', ['username' => $token->getUsername()]);
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            $this->logger->info('User has been authenticated successfully.', ['username' => method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername()]);
         }
 
         $this->tokenStorage->setToken($token);
diff --git a/src/Symfony/Component/Security/Http/Firewall/AbstractPreAuthenticatedListener.php b/src/Symfony/Component/Security/Http/Firewall/AbstractPreAuthenticatedListener.php
index 2cd905157b70b..7f8fcc1d82136 100644
--- a/src/Symfony/Component/Security/Http/Firewall/AbstractPreAuthenticatedListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/AbstractPreAuthenticatedListener.php
@@ -83,7 +83,8 @@ public function authenticate(RequestEvent $event)
         }
 
         if (null !== $token = $this->tokenStorage->getToken()) {
-            if ($token instanceof PreAuthenticatedToken && $this->providerKey == $token->getFirewallName() && $token->isAuthenticated() && $token->getUsername() === $user) {
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            if ($token instanceof PreAuthenticatedToken && $this->providerKey == $token->getFirewallName() && $token->isAuthenticated() && (method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername()) === $user) {
                 return;
             }
         }
diff --git a/src/Symfony/Component/Security/Http/Firewall/BasicAuthenticationListener.php b/src/Symfony/Component/Security/Http/Firewall/BasicAuthenticationListener.php
index a9ef56705fdd0..bf18dc168753b 100644
--- a/src/Symfony/Component/Security/Http/Firewall/BasicAuthenticationListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/BasicAuthenticationListener.php
@@ -73,7 +73,8 @@ public function authenticate(RequestEvent $event)
         }
 
         if (null !== $token = $this->tokenStorage->getToken()) {
-            if ($token instanceof UsernamePasswordToken && $token->isAuthenticated() && $token->getUsername() === $username) {
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            if ($token instanceof UsernamePasswordToken && $token->isAuthenticated() && (method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername()) === $username) {
                 return;
             }
         }
diff --git a/src/Symfony/Component/Security/Http/Firewall/ContextListener.php b/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
index a3511067ac9b6..6b3984687b7f4 100644
--- a/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/ContextListener.php
@@ -27,7 +27,7 @@
 use Symfony\Component\Security\Core\Authentication\Token\SwitchUserToken;
 use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Http\Event\DeauthenticatedEvent;
@@ -222,7 +222,8 @@ protected function refreshUser(TokenInterface $token): ?TokenInterface
                     $userDeauthenticated = true;
 
                     if (null !== $this->logger) {
-                        $this->logger->debug('Cannot refresh token because user has changed.', ['username' => $refreshedUser->getUsername(), 'provider' => \get_class($provider)]);
+                        // @deprecated since 5.3, change to $refreshedUser->getUserIdentifier() in 6.0
+                        $this->logger->debug('Cannot refresh token because user has changed.', ['username' => method_exists($refreshedUser, 'getUserIdentifier') ? $refreshedUser->getUserIdentifier() : $refreshedUser->getUsername(), 'provider' => \get_class($provider)]);
                     }
 
                     continue;
@@ -231,10 +232,12 @@ protected function refreshUser(TokenInterface $token): ?TokenInterface
                 $token->setUser($refreshedUser);
 
                 if (null !== $this->logger) {
-                    $context = ['provider' => \get_class($provider), 'username' => $refreshedUser->getUsername()];
+                    // @deprecated since 5.3, change to $refreshedUser->getUserIdentifier() in 6.0
+                    $context = ['provider' => \get_class($provider), 'username' => method_exists($refreshedUser, 'getUserIdentifier') ? $refreshedUser->getUserIdentifier() : $refreshedUser->getUsername()];
 
                     if ($token instanceof SwitchUserToken) {
-                        $context['impersonator_username'] = $token->getOriginalToken()->getUsername();
+                        // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+                        $context['impersonator_username'] = method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getOriginalToken()->getUsername();
                     }
 
                     $this->logger->debug('User was reloaded from a user provider.', $context);
@@ -243,9 +246,9 @@ protected function refreshUser(TokenInterface $token): ?TokenInterface
                 return $token;
             } catch (UnsupportedUserException $e) {
                 // let's try the next user provider
-            } catch (UsernameNotFoundException $e) {
+            } catch (UserNotFoundException $e) {
                 if (null !== $this->logger) {
-                    $this->logger->warning('Username could not be found in the selected user provider.', ['username' => $e->getUsername(), 'provider' => \get_class($provider)]);
+                    $this->logger->warning('Username could not be found in the selected user provider.', ['username' => method_exists($e, 'getUserIdentifier') ? $e->getUserIdentifier() : $e->getUsername(), 'provider' => \get_class($provider)]);
                 }
 
                 $userNotFoundByProvider = true;
diff --git a/src/Symfony/Component/Security/Http/Firewall/SwitchUserListener.php b/src/Symfony/Component/Security/Http/Firewall/SwitchUserListener.php
index 15b794c0a7503..b765b89da7d18 100644
--- a/src/Symfony/Component/Security/Http/Firewall/SwitchUserListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/SwitchUserListener.php
@@ -140,7 +140,8 @@ private function attemptSwitchUser(Request $request, string $username): ?TokenIn
         $originalToken = $this->getOriginalToken($token);
 
         if (null !== $originalToken) {
-            if ($token->getUsername() === $username) {
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            if ((method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername()) === $username) {
                 return $token;
             }
 
@@ -148,20 +149,27 @@ private function attemptSwitchUser(Request $request, string $username): ?TokenIn
             $token = $this->attemptExitUser($request);
         }
 
-        $currentUsername = $token->getUsername();
+        // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+        $currentUsername = method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername();
         $nonExistentUsername = '_'.md5(random_bytes(8).$username);
 
         // To protect against user enumeration via timing measurements
         // we always load both successfully and unsuccessfully
+        $methodName = 'loadUserByIdentifier';
+        if (!method_exists($this->provider, $methodName)) {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->provider));
+
+            $methodName = 'loadUserByUsername';
+        }
         try {
-            $user = $this->provider->loadUserByUsername($username);
+            $user = $this->provider->$methodName($username);
 
             try {
-                $this->provider->loadUserByUsername($nonExistentUsername);
+                $this->provider->$methodName($nonExistentUsername);
             } catch (\Exception $e) {
             }
         } catch (AuthenticationException $e) {
-            $this->provider->loadUserByUsername($currentUsername);
+            $this->provider->$methodName($currentUsername);
 
             throw $e;
         }
diff --git a/src/Symfony/Component/Security/Http/Firewall/UsernamePasswordJsonAuthenticationListener.php b/src/Symfony/Component/Security/Http/Firewall/UsernamePasswordJsonAuthenticationListener.php
index 9276e7f2ceb9e..5029cb95361c3 100644
--- a/src/Symfony/Component/Security/Http/Firewall/UsernamePasswordJsonAuthenticationListener.php
+++ b/src/Symfony/Component/Security/Http/Firewall/UsernamePasswordJsonAuthenticationListener.php
@@ -151,7 +151,8 @@ public function authenticate(RequestEvent $event)
     private function onSuccess(Request $request, TokenInterface $token): ?Response
     {
         if (null !== $this->logger) {
-            $this->logger->info('User has been authenticated successfully.', ['username' => $token->getUsername()]);
+            // @deprecated since 5.3, change to $token->getUserIdentifier() in 6.0
+            $this->logger->info('User has been authenticated successfully.', ['username' => method_exists($token, 'getUserIdentifier') ? $token->getUserIdentifier() : $token->getUsername()]);
         }
 
         $this->migrateSession($request, $token);
diff --git a/src/Symfony/Component/Security/Http/LoginLink/LoginLinkHandler.php b/src/Symfony/Component/Security/Http/LoginLink/LoginLinkHandler.php
index 1f4ddcc1ddc11..bfa67d6b6eb2a 100644
--- a/src/Symfony/Component/Security/Http/LoginLink/LoginLinkHandler.php
+++ b/src/Symfony/Component/Security/Http/LoginLink/LoginLinkHandler.php
@@ -15,7 +15,7 @@
 use Symfony\Component\PropertyAccess\PropertyAccessorInterface;
 use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 use Symfony\Component\Routing\RequestContext;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Http\LoginLink\Exception\ExpiredLoginLinkException;
@@ -56,7 +56,8 @@ public function createLoginLink(UserInterface $user, Request $request = null): L
 
         $expires = $expiresAt->format('U');
         $parameters = [
-            'user' => $user->getUsername(),
+            // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+            'user' => method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(),
             'expires' => $expires,
             'hash' => $this->computeSignatureHash($user, $expires),
         ];
@@ -83,11 +84,18 @@ public function createLoginLink(UserInterface $user, Request $request = null): L
 
     public function consumeLoginLink(Request $request): UserInterface
     {
-        $username = $request->get('user');
+        $userIdentifier = $request->get('user');
 
         try {
-            $user = $this->userProvider->loadUserByUsername($username);
-        } catch (UsernameNotFoundException $exception) {
+            // @deprecated since 5.3, change to $this->userProvider->loadUserByIdentifier() in 6.0
+            if (method_exists($this->userProvider, 'loadUserByIdentifier')) {
+                $user = $this->userProvider->loadUserByIdentifier($userIdentifier);
+            } else {
+                trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($this->userProvider));
+
+                $user = $this->userProvider->loadUserByUsername($userIdentifier);
+            }
+        } catch (UserNotFoundException $exception) {
             throw new InvalidLoginLinkException('User not found.', 0, $exception);
         }
 
@@ -115,7 +123,8 @@ public function consumeLoginLink(Request $request): UserInterface
 
     private function computeSignatureHash(UserInterface $user, int $expires): string
     {
-        $signatureFields = [base64_encode($user->getUsername()), $expires];
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        $signatureFields = [base64_encode(method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername()), $expires];
 
         foreach ($this->signatureProperties as $property) {
             $value = $this->propertyAccessor->getValue($user, $property) ?? '';
diff --git a/src/Symfony/Component/Security/Http/RememberMe/AbstractRememberMeServices.php b/src/Symfony/Component/Security/Http/RememberMe/AbstractRememberMeServices.php
index 295e7d80a4951..aa2ab934d13b9 100644
--- a/src/Symfony/Component/Security/Http/RememberMe/AbstractRememberMeServices.php
+++ b/src/Symfony/Component/Security/Http/RememberMe/AbstractRememberMeServices.php
@@ -20,7 +20,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\CookieTheftException;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Http\Logout\LogoutHandlerInterface;
@@ -40,6 +40,8 @@ abstract class AbstractRememberMeServices implements RememberMeServicesInterface
         'secure' => false,
         'httponly' => true,
         'samesite' => null,
+        'path' => null,
+        'domain' => null,
     ];
     private $firewallName;
     private $secret;
@@ -128,7 +130,7 @@ final public function autoLogin(Request $request): ?TokenInterface
             $this->loginFail($request, $e);
 
             throw $e;
-        } catch (UsernameNotFoundException $e) {
+        } catch (UserNotFoundException $e) {
             if (null !== $this->logger) {
                 $this->logger->info('User for remember-me cookie not found.', ['exception' => $e]);
             }
diff --git a/src/Symfony/Component/Security/Http/RememberMe/PersistentTokenBasedRememberMeServices.php b/src/Symfony/Component/Security/Http/RememberMe/PersistentTokenBasedRememberMeServices.php
index 5711f0f5acca8..c7376faa910d9 100644
--- a/src/Symfony/Component/Security/Http/RememberMe/PersistentTokenBasedRememberMeServices.php
+++ b/src/Symfony/Component/Security/Http/RememberMe/PersistentTokenBasedRememberMeServices.php
@@ -93,7 +93,24 @@ protected function processAutoLoginCookie(array $cookieParts, Request $request)
             )
         );
 
-        return $this->getUserProvider($persistentToken->getClass())->loadUserByUsername($persistentToken->getUsername());
+        $userProvider = $this->getUserProvider($persistentToken->getClass());
+        // @deprecated since 5.3, change to $persistentToken->getUserIdentifier() in 6.0
+        if (method_exists($persistentToken, 'getUserIdentifier')) {
+            $userIdentifier = $persistentToken->getUserIdentifier();
+        } else {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "getUserIdentifier()" in persistent token "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($persistentToken));
+
+            $userIdentifier = $persistentToken->getUsername();
+        }
+
+        // @deprecated since 5.3, change to $userProvider->loadUserByIdentifier() in 6.0
+        if (method_exists($userProvider, 'loadUserByIdentifier')) {
+            return $userProvider->loadUserByIdentifier($userIdentifier);
+        } else {
+            trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($userProvider));
+
+            return $userProvider->loadUserByUsername($userIdentifier);
+        }
     }
 
     /**
@@ -107,7 +124,8 @@ protected function onLoginSuccess(Request $request, Response $response, TokenInt
         $this->tokenProvider->createNewToken(
             new PersistentToken(
                 \get_class($user = $token->getUser()),
-                $user->getUsername(),
+                // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+                method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(),
                 $series,
                 $this->generateHash($tokenValue),
                 new \DateTime()
diff --git a/src/Symfony/Component/Security/Http/RememberMe/TokenBasedRememberMeServices.php b/src/Symfony/Component/Security/Http/RememberMe/TokenBasedRememberMeServices.php
index 80e0b058157e2..a6e79f42e56d7 100644
--- a/src/Symfony/Component/Security/Http/RememberMe/TokenBasedRememberMeServices.php
+++ b/src/Symfony/Component/Security/Http/RememberMe/TokenBasedRememberMeServices.php
@@ -35,12 +35,20 @@ protected function processAutoLoginCookie(array $cookieParts, Request $request)
             throw new AuthenticationException('The cookie is invalid.');
         }
 
-        [$class, $username, $expires, $hash] = $cookieParts;
-        if (false === $username = base64_decode($username, true)) {
-            throw new AuthenticationException('$username contains a character from outside the base64 alphabet.');
+        [$class, $userIdentifier, $expires, $hash] = $cookieParts;
+        if (false === $userIdentifier = base64_decode($userIdentifier, true)) {
+            throw new AuthenticationException('$userIdentifier contains a character from outside the base64 alphabet.');
         }
         try {
-            $user = $this->getUserProvider($class)->loadUserByUsername($username);
+            $userProvider = $this->getUserProvider($class);
+            // @deprecated since 5.3, change to $userProvider->loadUserByIdentifier() in 6.0
+            if (method_exists($userProvider, 'loadUserByIdentifier')) {
+                $user = $userProvider->loadUserByIdentifier($userIdentifier);
+            } else {
+                trigger_deprecation('symfony/security-core', '5.3', 'Not implementing method "loadUserByIdentifier()" in user provider "%s" is deprecated. This method will replace "loadUserByUsername()" in Symfony 6.0.', get_debug_type($userProvider));
+
+                $user = $userProvider->loadUserByUsername($userIdentifier);
+            }
         } catch (\Exception $e) {
             if (!$e instanceof AuthenticationException) {
                 $e = new AuthenticationException($e->getMessage(), $e->getCode(), $e);
@@ -53,7 +61,7 @@ protected function processAutoLoginCookie(array $cookieParts, Request $request)
             throw new \RuntimeException(sprintf('The UserProviderInterface implementation must return an instance of UserInterface, but returned "%s".', get_debug_type($user)));
         }
 
-        if (true !== hash_equals($this->generateCookieHash($class, $username, $expires, $user->getPassword()), $hash)) {
+        if (true !== hash_equals($this->generateCookieHash($class, $userIdentifier, $expires, $user->getPassword()), $hash)) {
             throw new AuthenticationException('The cookie\'s hash is invalid.');
         }
 
@@ -71,7 +79,8 @@ protected function onLoginSuccess(Request $request, Response $response, TokenInt
     {
         $user = $token->getUser();
         $expires = time() + $this->options['lifetime'];
-        $value = $this->generateCookieValue(\get_class($user), $user->getUsername(), $expires, $user->getPassword());
+        // @deprecated since 5.3, change to $user->getUserIdentifier() in 6.0
+        $value = $this->generateCookieValue(\get_class($user), method_exists($user, 'getUserIdentifier') ? $user->getUserIdentifier() : $user->getUsername(), $expires, $user->getPassword());
 
         $response->headers->setCookie(
             new Cookie(
@@ -96,15 +105,15 @@ protected function onLoginSuccess(Request $request, Response $response, TokenInt
      *
      * @return string
      */
-    protected function generateCookieValue(string $class, string $username, int $expires, ?string $password)
+    protected function generateCookieValue(string $class, string $userIdentifier, int $expires, ?string $password)
     {
-        // $username is encoded because it might contain COOKIE_DELIMITER,
+        // $userIdentifier is encoded because it might contain COOKIE_DELIMITER,
         // we assume other values don't
         return $this->encodeCookie([
             $class,
-            base64_encode($username),
+            base64_encode($userIdentifier),
             $expires,
-            $this->generateCookieHash($class, $username, $expires, $password),
+            $this->generateCookieHash($class, $userIdentifier, $expires, $password),
         ]);
     }
 
@@ -116,8 +125,8 @@ protected function generateCookieValue(string $class, string $username, int $exp
      *
      * @return string
      */
-    protected function generateCookieHash(string $class, string $username, int $expires, ?string $password)
+    protected function generateCookieHash(string $class, string $userIdentifier, int $expires, ?string $password)
     {
-        return hash_hmac('sha256', $class.self::COOKIE_DELIMITER.$username.self::COOKIE_DELIMITER.$expires.self::COOKIE_DELIMITER.$password, $this->getSecret());
+        return hash_hmac('sha256', $class.self::COOKIE_DELIMITER.$userIdentifier.self::COOKIE_DELIMITER.$expires.self::COOKIE_DELIMITER.$password, $this->getSecret());
     }
 }
diff --git a/src/Symfony/Component/Security/Http/Tests/Authentication/AuthenticatorManagerTest.php b/src/Symfony/Component/Security/Http/Tests/Authentication/AuthenticatorManagerTest.php
index 969243f83e862..89f0ddf07a10a 100644
--- a/src/Symfony/Component/Security/Http/Tests/Authentication/AuthenticatorManagerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Authentication/AuthenticatorManagerTest.php
@@ -44,6 +44,7 @@ protected function setUp(): void
         $this->request = new Request();
         $this->user = new InMemoryUser('wouter', null);
         $this->token = $this->createMock(TokenInterface::class);
+        $this->token->expects($this->any())->method('getUser')->willReturn($this->user);
         $this->response = $this->createMock(Response::class);
     }
 
@@ -166,6 +167,7 @@ public function testAuthenticateRequestCanModifyTokenFromEvent()
         $authenticator->expects($this->any())->method('createAuthenticatedToken')->willReturn($this->token);
 
         $modifiedToken = $this->createMock(TokenInterface::class);
+        $modifiedToken->expects($this->any())->method('getUser')->willReturn($this->user);
         $listenerCalled = false;
         $this->eventDispatcher->addListener(AuthenticationTokenCreatedEvent::class, function (AuthenticationTokenCreatedEvent $event) use (&$listenerCalled, $modifiedToken) {
             $event->setAuthenticatedToken($modifiedToken);
@@ -198,6 +200,7 @@ public function testAuthenticateUserCanModifyTokenFromEvent()
         $authenticator->expects($this->any())->method('onAuthenticationSuccess')->willReturn($this->response);
 
         $modifiedToken = $this->createMock(TokenInterface::class);
+        $modifiedToken->expects($this->any())->method('getUser')->willReturn($this->user);
         $listenerCalled = false;
         $this->eventDispatcher->addListener(AuthenticationTokenCreatedEvent::class, function (AuthenticationTokenCreatedEvent $event) use (&$listenerCalled, $modifiedToken) {
             $event->setAuthenticatedToken($modifiedToken);
diff --git a/src/Symfony/Component/Security/Http/Tests/Authenticator/JsonLoginAuthenticatorTest.php b/src/Symfony/Component/Security/Http/Tests/Authenticator/JsonLoginAuthenticatorTest.php
index 1f86aa648abe7..47e02689ead93 100644
--- a/src/Symfony/Component/Security/Http/Tests/Authenticator/JsonLoginAuthenticatorTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Authenticator/JsonLoginAuthenticatorTest.php
@@ -17,7 +17,7 @@
 use Symfony\Component\Security\Core\Exception\AuthenticationException;
 use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 use Symfony\Component\Security\Core\Security;
-use Symfony\Component\Security\Core\User\UserProviderInterface;
+use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Http\Authenticator\JsonLoginAuthenticator;
 use Symfony\Component\Security\Http\Authenticator\Passport\Credentials\PasswordCredentials;
 use Symfony\Component\Security\Http\HttpUtils;
@@ -32,7 +32,7 @@ class JsonLoginAuthenticatorTest extends TestCase
 
     protected function setUp(): void
     {
-        $this->userProvider = $this->createMock(UserProviderInterface::class);
+        $this->userProvider = new InMemoryUserProvider();
     }
 
     /**
diff --git a/src/Symfony/Component/Security/Http/Tests/Authenticator/X509AuthenticatorTest.php b/src/Symfony/Component/Security/Http/Tests/Authenticator/X509AuthenticatorTest.php
index 0acf418068078..7ee6aaa973894 100644
--- a/src/Symfony/Component/Security/Http/Tests/Authenticator/X509AuthenticatorTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Authenticator/X509AuthenticatorTest.php
@@ -48,7 +48,7 @@ public function testAuthentication($username, $credentials)
         $this->userProvider->createUser(new InMemoryUser($username, null));
 
         $passport = $this->authenticator->authenticate($request);
-        $this->assertEquals($username, $passport->getUser()->getUsername());
+        $this->assertEquals($username, $passport->getUser()->getUserIdentifier());
     }
 
     public static function provideServerVars()
@@ -69,7 +69,7 @@ public function testAuthenticationNoUser($emailAddress, $credentials)
         $this->userProvider->createUser(new InMemoryUser($emailAddress, null));
 
         $passport = $this->authenticator->authenticate($request);
-        $this->assertEquals($emailAddress, $passport->getUser()->getUsername());
+        $this->assertEquals($emailAddress, $passport->getUser()->getUserIdentifier());
     }
 
     public static function provideServerVarsNoUser()
@@ -102,7 +102,7 @@ public function testAuthenticationCustomUserKey()
         $this->userProvider->createUser(new InMemoryUser('TheUser', null));
 
         $passport = $this->authenticator->authenticate($request);
-        $this->assertEquals('TheUser', $passport->getUser()->getUsername());
+        $this->assertEquals('TheUser', $passport->getUser()->getUserIdentifier());
     }
 
     public function testAuthenticationCustomCredentialsKey()
@@ -117,7 +117,7 @@ public function testAuthenticationCustomCredentialsKey()
         $this->userProvider->createUser(new InMemoryUser('cert@example.com', null));
 
         $passport = $authenticator->authenticate($request);
-        $this->assertEquals('cert@example.com', $passport->getUser()->getUsername());
+        $this->assertEquals('cert@example.com', $passport->getUser()->getUserIdentifier());
     }
 
     private function createRequest(array $server)
diff --git a/src/Symfony/Component/Security/Http/Tests/EventListener/LoginThrottlingListenerTest.php b/src/Symfony/Component/Security/Http/Tests/EventListener/LoginThrottlingListenerTest.php
index 1374fc39b0a4a..bd53763273b50 100644
--- a/src/Symfony/Component/Security/Http/Tests/EventListener/LoginThrottlingListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/EventListener/LoginThrottlingListenerTest.php
@@ -90,7 +90,7 @@ private function createPassport($username)
     private function createLoginSuccessfulEvent($passport, $username = 'wouter')
     {
         $token = $this->createMock(TokenInterface::class);
-        $token->expects($this->any())->method('getUsername')->willReturn($username);
+        $token->expects($this->any())->method('getUserIdentifier')->willReturn($username);
 
         return new LoginSuccessEvent($this->createMock(AuthenticatorInterface::class), $passport, $token, $this->requestStack->getCurrentRequest(), null, 'main');
     }
diff --git a/src/Symfony/Component/Security/Http/Tests/EventListener/PasswordMigratingListenerTest.php b/src/Symfony/Component/Security/Http/Tests/EventListener/PasswordMigratingListenerTest.php
index b347276b0f673..e07b502a80c35 100644
--- a/src/Symfony/Component/Security/Http/Tests/EventListener/PasswordMigratingListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/EventListener/PasswordMigratingListenerTest.php
@@ -99,14 +99,14 @@ public function testUpgradeWithUpgrader()
     public function testUpgradeWithoutUpgrader()
     {
         $userLoader = $this->getMockForAbstractClass(TestMigratingUserProvider::class);
-        $userLoader->expects($this->any())->method('loadUserByUsername')->willReturn($this->user);
+        $userLoader->expects($this->any())->method('loadUserByIdentifier')->willReturn($this->user);
 
         $userLoader->expects($this->once())
             ->method('upgradePassword')
             ->with($this->user, 'new-hash')
         ;
 
-        $event = $this->createEvent(new SelfValidatingPassport(new UserBadge('test', [$userLoader, 'loadUserByUsername']), [new PasswordUpgradeBadge('pa$$word')]));
+        $event = $this->createEvent(new SelfValidatingPassport(new UserBadge('test', [$userLoader, 'loadUserByIdentifier']), [new PasswordUpgradeBadge('pa$$word')]));
         $this->listener->onLoginSuccess($event);
     }
 
@@ -124,6 +124,7 @@ private function createEvent(PassportInterface $passport)
 abstract class TestMigratingUserProvider implements UserProviderInterface, PasswordUpgraderInterface
 {
     abstract public function upgradePassword(PasswordAuthenticatedUserInterface $user, string $newHashedPassword): void;
+    abstract public function loadUserByIdentifier(string $identifier): UserInterface;
 }
 
 abstract class TestPasswordAuthenticatedUser implements UserInterface, PasswordAuthenticatedUserInterface
diff --git a/src/Symfony/Component/Security/Http/Tests/EventListener/UserProviderListenerTest.php b/src/Symfony/Component/Security/Http/Tests/EventListener/UserProviderListenerTest.php
index 8901b95658896..1f5a65ac0b926 100644
--- a/src/Symfony/Component/Security/Http/Tests/EventListener/UserProviderListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/EventListener/UserProviderListenerTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Security\Http\Tests\EventListener;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Http\Authenticator\AuthenticatorInterface;
@@ -22,6 +23,8 @@
 
 class UserProviderListenerTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     private $userProvider;
     private $listener;
 
@@ -38,7 +41,7 @@ public function testSetUserProvider()
         $this->listener->checkPassport(new CheckPassportEvent($this->createMock(AuthenticatorInterface::class), $passport));
 
         $badge = $passport->getBadge(UserBadge::class);
-        $this->assertEquals([$this->userProvider, 'loadUserByUsername'], $badge->getUserLoader());
+        $this->assertEquals([$this->userProvider, 'loadUserByIdentifier'], $badge->getUserLoader());
 
         $user = new InMemoryUser('wouter', null);
         $this->userProvider->createUser($user);
diff --git a/src/Symfony/Component/Security/Http/Tests/Firewall/AccessListenerTest.php b/src/Symfony/Component/Security/Http/Tests/Firewall/AccessListenerTest.php
index 525456b59ebd4..aac67379b4e7f 100644
--- a/src/Symfony/Component/Security/Http/Tests/Firewall/AccessListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Firewall/AccessListenerTest.php
@@ -338,12 +338,7 @@ public function testHandleMWithultipleAttributesShouldBeHandledAsAnd()
             ->willReturn([['foo' => 'bar', 'bar' => 'baz'], null])
         ;
 
-        $authenticatedToken = $this->createMock(TokenInterface::class);
-        $authenticatedToken
-            ->expects($this->any())
-            ->method('isAuthenticated')
-            ->willReturn(true)
-        ;
+        $authenticatedToken = new UsernamePasswordToken('test', 'test', 'test', ['ROLE_USER']);
 
         $tokenStorage = new TokenStorage();
         $tokenStorage->setToken($authenticatedToken);
diff --git a/src/Symfony/Component/Security/Http/Tests/Firewall/ContextListenerTest.php b/src/Symfony/Component/Security/Http/Tests/Firewall/ContextListenerTest.php
index a48c8f7cb0515..6a612cdb80928 100644
--- a/src/Symfony/Component/Security/Http/Tests/Firewall/ContextListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Firewall/ContextListenerTest.php
@@ -31,7 +31,7 @@
 use Symfony\Component\Security\Core\Authentication\Token\Storage\UsageTrackingTokenStorage;
 use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
 use Symfony\Component\Security\Core\Exception\UnsupportedUserException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
@@ -69,7 +69,7 @@ public function testOnKernelResponseWillAddSession()
 
         $token = unserialize($session->get('_security_session'));
         $this->assertInstanceOf(UsernamePasswordToken::class, $token);
-        $this->assertEquals('test1', $token->getUsername());
+        $this->assertEquals('test1', $token->getUserIdentifier());
     }
 
     public function testOnKernelResponseWillReplaceSession()
@@ -81,7 +81,7 @@ public function testOnKernelResponseWillReplaceSession()
 
         $token = unserialize($session->get('_security_session'));
         $this->assertInstanceOf(UsernamePasswordToken::class, $token);
-        $this->assertEquals('test1', $token->getUsername());
+        $this->assertEquals('test1', $token->getUserIdentifier());
     }
 
     public function testOnKernelResponseWillRemoveSession()
@@ -478,7 +478,12 @@ public function __construct($throwsUnsupportedException)
 
     public function loadUserByUsername($username): UserInterface
     {
-        throw new UsernameNotFoundException();
+        throw new UserNotFoundException();
+    }
+
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+        throw new UserNotFoundException();
     }
 
     public function refreshUser(UserInterface $user): UserInterface
@@ -509,6 +514,10 @@ public function loadUserByUsername($username): UserInterface
     {
     }
 
+    public function loadUserByIdentifier(string $identifier): UserInterface
+    {
+    }
+
     public function refreshUser(UserInterface $user): UserInterface
     {
         if (!$user instanceof InMemoryUser) {
@@ -516,7 +525,7 @@ public function refreshUser(UserInterface $user): UserInterface
         }
 
         if (null === $this->refreshedUser) {
-            throw new UsernameNotFoundException();
+            throw new UserNotFoundException();
         }
 
         return $this->refreshedUser;
diff --git a/src/Symfony/Component/Security/Http/Tests/Firewall/SwitchUserListenerTest.php b/src/Symfony/Component/Security/Http/Tests/Firewall/SwitchUserListenerTest.php
index 9e5adefab7036..64cc07e15d6b3 100644
--- a/src/Symfony/Component/Security/Http/Tests/Firewall/SwitchUserListenerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/Firewall/SwitchUserListenerTest.php
@@ -22,7 +22,6 @@
 use Symfony\Component\Security\Core\Authorization\AccessDecisionManagerInterface;
 use Symfony\Component\Security\Core\Exception\AccessDeniedException;
 use Symfony\Component\Security\Core\Exception\AuthenticationCredentialsNotFoundException;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
 use Symfony\Component\Security\Core\User\InMemoryUser;
 use Symfony\Component\Security\Core\User\InMemoryUserProvider;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
@@ -200,11 +199,11 @@ public function testSwitchUser()
         $this->request->query->set('_switch_user', 'kuba');
 
         $this->accessDecisionManager->expects($this->once())
-            ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $this->callback(function ($user) { return 'kuba' === $user->getUsername(); }))
+            ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); }))
             ->willReturn(true);
 
         $this->userChecker->expects($this->once())
-            ->method('checkPostAuth')->with($this->callback(function ($user) { return 'kuba' === $user->getUsername(); }));
+            ->method('checkPostAuth')->with($this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); }));
 
         $listener = new SwitchUserListener($this->tokenStorage, $this->userProvider, $this->userChecker, 'provider123', $this->accessDecisionManager);
         $listener($this->event);
@@ -222,11 +221,9 @@ public function testSwitchUserAlreadySwitched()
         $tokenStorage = new TokenStorage();
         $tokenStorage->setToken($alreadySwitchedToken);
 
-        $targetUser = new InMemoryUser('kuba', 'password', ['ROLE_FOO', 'ROLE_BAR']);
-
         $this->request->query->set('_switch_user', 'kuba');
 
-        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUsername(); });
+        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); });
         $this->accessDecisionManager->expects($this->once())
             ->method('decide')->with($originalToken, ['ROLE_ALLOWED_TO_SWITCH'], $targetsUser)
             ->willReturn(true);
@@ -240,7 +237,7 @@ public function testSwitchUserAlreadySwitched()
         $this->assertSame([], $this->request->query->all());
         $this->assertSame('', $this->request->server->get('QUERY_STRING'));
         $this->assertInstanceOf(SwitchUserToken::class, $tokenStorage->getToken());
-        $this->assertSame('kuba', $tokenStorage->getToken()->getUsername());
+        $this->assertSame('kuba', $tokenStorage->getToken()->getUserIdentifier());
         $this->assertSame($originalToken, $tokenStorage->getToken()->getOriginalToken());
     }
 
@@ -279,7 +276,7 @@ public function testSwitchUserKeepsOtherQueryStringParameters()
             'section' => 2,
         ]);
 
-        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUsername(); });
+        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); });
         $this->accessDecisionManager->expects($this->once())
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $targetsUser)
             ->willReturn(true);
@@ -306,7 +303,7 @@ public function testSwitchUserWithReplacedToken()
         $this->request->query->set('_switch_user', 'kuba');
 
         $this->accessDecisionManager->expects($this->any())
-            ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $this->callback(function ($user) { return 'kuba' === $user->getUsername(); }))
+            ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); }))
             ->willReturn(true);
 
         $dispatcher = $this->createMock(EventDispatcherInterface::class);
@@ -314,8 +311,8 @@ public function testSwitchUserWithReplacedToken()
             ->expects($this->once())
             ->method('dispatch')
             ->with(
-                $this->callback(function (SwitchUserEvent $event) use ($replacedToken) {
-                    if ('kuba' !== $event->getTargetUser()->getUsername()) {
+                $this->callback(function (SwitchUserEvent $event) use ($replacedToken, $user) {
+                    if ('kuba' !== $event->getTargetUser()->getUserIdentifier()) {
                         return false;
                     }
                     $event->setToken($replacedToken);
@@ -343,12 +340,11 @@ public function testSwitchUserThrowsAuthenticationExceptionIfNoCurrentToken()
     public function testSwitchUserStateless()
     {
         $token = new UsernamePasswordToken('username', '', 'key', ['ROLE_FOO']);
-        $user = new InMemoryUser('username', 'password', []);
 
         $this->tokenStorage->setToken($token);
         $this->request->query->set('_switch_user', 'kuba');
 
-        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUsername(); });
+        $targetsUser = $this->callback(function ($user) { return 'kuba' === $user->getUserIdentifier(); });
         $this->accessDecisionManager->expects($this->once())
             ->method('decide')->with($token, ['ROLE_ALLOWED_TO_SWITCH'], $targetsUser)
             ->willReturn(true);
diff --git a/src/Symfony/Component/Security/Http/Tests/LoginLink/LoginLinkHandlerTest.php b/src/Symfony/Component/Security/Http/Tests/LoginLink/LoginLinkHandlerTest.php
index 9d063f6630290..74050396ac077 100644
--- a/src/Symfony/Component/Security/Http/Tests/LoginLink/LoginLinkHandlerTest.php
+++ b/src/Symfony/Component/Security/Http/Tests/LoginLink/LoginLinkHandlerTest.php
@@ -18,7 +18,7 @@
 use Symfony\Component\PropertyAccess\PropertyAccessorInterface;
 use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 use Symfony\Component\Routing\RequestContext;
-use Symfony\Component\Security\Core\Exception\UsernameNotFoundException;
+use Symfony\Component\Security\Core\Exception\UserNotFoundException;
 use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Http\LoginLink\Exception\ExpiredLoginLinkException;
@@ -208,13 +208,13 @@ public function createUser(TestLoginLinkHandlerUser $user): void
         $this->users[$user->getUsername()] = $user;
     }
 
-    public function loadUserByUsername(string $username): TestLoginLinkHandlerUser
+    public function loadUserByIdentifier(string $userIdentifier): TestLoginLinkHandlerUser
     {
-        if (!isset($this->users[$username])) {
-            throw new UsernameNotFoundException();
+        if (!isset($this->users[$userIdentifier])) {
+            throw new UserNotFoundException();
         }
 
-        return clone $this->users[$username];
+        return clone $this->users[$userIdentifier];
     }
 
     public function refreshUser(UserInterface $user)
@@ -263,6 +263,11 @@ public function getUsername()
         return $this->username;
     }
 
+    public function getUserIdentifier()
+    {
+        return $this->username;
+    }
+
     public function eraseCredentials()
     {
     }