10000 Make SCRAM iteration count configurable · postgrespro/postgres@b577743 · GitHub
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Make SCRAM iteration count configurable
Replace the hardcoded value with a GUC such that the iteration count can be raised in order to increase protection against brute-force attacks. The hardcoded value for SCRAM iteration count was defined to be 4096, which is taken from RFC 7677, so set the default for the GUC to 4096 to match. In RFC 7677 the recommendation is at least 15000 iterations but 4096 is listed as a SHOULD requirement given that it's estimated to yield a 0.5s processing time on a mobile handset of the time of RFC writing (late 2015). Raising the iteration count of SCRAM will make stored passwords more resilient to brute-force attacks at a higher computational cost during connection establishment. Lowering the count will reduce computational overhead during connections at the tradeoff of reducing strength against brute-force attacks. There are however platforms where even a modest iteration count yields a too high computational overhead, with weaker password encryption schemes chosen as a result. In these situations, SCRAM with a very low iteration count still gives benefits over weaker schemes like md5, so we allow the iteration count to be set to one at the low end. The new GUC is intentionally generically named such that it can be made to support future SCRAM standards should they emerge. At that point the value can be made into key:value pairs with an undefined key as a default which will be backwards compatible with this. Reviewed-by: Michael Paquier <michael@paquier.xyz> Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org> Discussion: https://postgr.es/m/F72E7BC7-189F-4B17-BF47-9735EB72C364@yesql.se
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16 files changed

+94
-10
lines changed

16 files changed

+94
-10
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‎doc/src/sgml/config.sgml

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Original file line numberDiff line numberDiff line change
@@ -1132,6 +1132,26 @@ include_dir 'conf.d'
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>scram_iterations</varname> (<type>integer</type>)
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<indexterm>
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<primary><varname>scram_iterations</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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The number of computational iterations to be performed when encrypting
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a password using SCRAM-SHA-256. The default is <literal>4096</literal>.
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A higher number of iterations provides additional protection against
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brute-force attacks on stored passwords, but makes authentication
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slower. Changing the value has no effect on existing passwords
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encrypted with SCRAM-SHA-256 as the iteration count is fixed at the
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time of encryption. In order to make use of a changed value, a new
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password must be set.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
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<term><varname>krb_server_keyfile</varname> (<type>string</type>)
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<indexterm>

‎src/backend/libpq/auth-scram.c

Lines changed: 7 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -191,6 +191,11 @@ static char *scram_mock_salt(const char *username,
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pg_cryptohash_type hash_type,
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int key_length);
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/*
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* The number of iterations to use when generating new secrets.
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*/
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int scram_sha_256_iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
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/*
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* Get a list of SASL mechanisms that this module supports.
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*
@@ -496,7 +501,7 @@ pg_be_scram_build_secret(const char *password)
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result = scram_build_secret(PG_SHA256, SCRAM_SHA_256_KEY_LEN,
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saltbuf, SCRAM_DEFAULT_SALT_LEN,
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SCRAM_DEFAULT_ITERATIONS, password,
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scram_sha_256_iterations, password,
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&errstr);
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if (prep_password)
@@ -717,7 +722,7 @@ mock_scram_secret(const char *username, pg_cryptohash_type *hash_type,
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encoded_salt[encoded_len] = '\0';
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*salt = encoded_salt;
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*iterations = SCRAM_DEFAULT_ITERATIONS;
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*iterations = SCRAM_SHA_256_DEFAULT_ITERATIONS;
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/* StoredKey and ServerKey are not used in a doomed authentication */
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memset(stored_key, 0, SCRAM_MAX_KEY_LEN);

‎src/backend/utils/misc/guc_tables.c

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Original file line numberDiff line numberDiff line change
@@ -41,9 +41,11 @@
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#include "commands/trigger.h"
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#include "commands/user.h"
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#include "commands/vacuum.h"
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#include "common/scram-common.h"
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#include "jit/jit.h"
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#include "libpq/auth.h"
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#include "libpq/libpq.h"
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#include "libpq/scram.h"
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#include "nodes/queryjumble.h"
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#include "optimizer/cost.h"
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#include "optimizer/geqo.h"
@@ -3468,6 +3470,17 @@ struct config_int ConfigureNamesInt[] =
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NULL, NULL, NULL
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},
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{
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{"scram_iterations", PGC_USERSET, CONN_AUTH_AUTH,
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gettext_noop("Sets the iteration count for SCRAM secret generation."),
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NULL,
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GUC_REPORT
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},
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&scram_sha_256_iterations,
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SCRAM_SHA_256_DEFAULT_ITERATIONS, 1, INT_MAX,
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NULL, NULL, NULL
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},
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/* End-of-list marker */
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{
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{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL

‎src/backend/utils/misc/postgresql.conf.sample

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@@ -95,6 +95,7 @@
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#authentication_timeout = 1min # 1s-600s
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#password_encryption = scram-sha-256 # scram-sha-256 or md5
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#scram_iterations = 4096
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#db_user_namespace = off
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# GSSAPI using Kerberos

‎src/common/scram-common.c

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -214,8 +214,7 @@ scram_build_secret(pg_cryptohash_type hash_type, int key_length,
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/* Only this hash method is supported currently */
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Assert(hash_type == PG_SHA256);
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if (iterations <= 0)
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iterations = SCRAM_DEFAULT_ITERATIONS;
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Assert(iterations > 0);
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/* Calculate StoredKey and ServerKey */
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if (scram_SaltedPassword(password, hash_type, key_length,

‎src/include/common/scram-common.h

Lines changed: 1 addition & 1 deletion
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@@ -47,7 +47,7 @@
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* Default number of iterations when generating secret. Should be at least
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* 4096 per RFC 7677.
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*/
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#define SCRAM_DEFAULT_ITERATIONS 4096
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#define SCRAM_SHA_256_DEFAULT_ITERATIONS 4096
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extern int scram_SaltedPassword(const char *password,
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pg_cryptohash_type hash_type, int key_length,

‎src/include/libpq/scram.h

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@@ -18,6 +18,9 @@
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#include "libpq/libpq-be.h"
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#include "libpq/sasl.h"
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/* Number of iterations when generating new secrets */
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extern PGDLLIMPORT int scram_sha_256_iterations;
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/* SASL implementation callbacks */
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extern PGDLLIMPORT const pg_be_sasl_mech pg_be_scram_mech;
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‎src/interfaces/libpq/fe-auth-scram.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -895,7 +895,7 @@ verify_server_signature(fe_scram_state *state, bool *match,
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* error details.
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*/
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char *
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pg_fe_scram_build_secret(const char *password, const char **errstr)
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pg_fe_scram_build_secret(const char *password, int iterations, const char **errstr)
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{
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char *prep_password;
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pg_saslprep_rc rc;
@@ -927,7 +927,7 @@ pg_fe_scram_build_secret(const char *password, const char **errstr)
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result = scram_build_secret(PG_SHA256, SCRAM_SHA_256_KEY_LEN, saltbuf,
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SCRAM_DEFAULT_SALT_LEN,
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SCRAM_DEFAULT_ITERATIONS, password,
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iterations, password,
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errstr);
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free(prep_password);

‎src/interfaces/libpq/fe-auth.c

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1341,7 +1341,9 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
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{
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const char *errstr = NULL;
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crypt_pwd = pg_fe_scram_build_secret(passwd, &errstr);
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crypt_pwd = pg_fe_scram_build_secret(passwd,
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conn->scram_sha_256_iterations,
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&errstr);
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if (!crypt_pwd)
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libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
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}

‎src/interfaces/libpq/fe-auth.h

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@@ -26,6 +26,7 @@ extern char *pg_fe_getauthname(PQExpBuffer errorMessage);
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/* Mechanisms in fe-auth-scram.c */
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extern const pg_fe_sasl_mech pg_scram_mech;
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extern char *pg_fe_scram_build_secret(const char *password,
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int iterations,
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const char **errstr);
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#endif /* FE_AUTH_H */

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