cmake: warn for not using sha1dc

ethomson · ethomson · commit b027a22d377d · 2024-12-28T09:49:28.000Z
git's hash algorithm is sha1dc, it is not sha1. Per Linus:

&gt; Honestly, git has effectively already moved from SHA1 to SHA1DC.
&gt;
&gt; So the actual known attack and weakness of SHA1 should simply not be
&gt; part of the discussion for the next hash. You can basically say "we're
&gt; _already_ on the second hash, we just picked one that was so
&gt; compatible with SHA1 that nobody even really noticed.

Warn users who try to compile with SHA1 instead of SHA1DC.
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -150,3 +150,9 @@ endif()
 
 feature_summary(WHAT ENABLED_FEATURES DESCRIPTION "Enabled features:")
 feature_summary(WHAT DISABLED_FEATURES DESCRIPTION "Disabled features:")
+
+# warn for not using sha1dc
+
+foreach(WARNING ${WARNINGS})
+	message(WARNING ${WARNING})
+endforeach()
diff --git a/cmake/SelectHashes.cmake b/cmake/SelectHashes.cmake
@@ -112,3 +112,10 @@ endif()
 
 add_feature_info(SHA1 ON "using ${USE_SHA1}")
 add_feature_info(SHA256 ON "using ${USE_SHA256}")
+
+# warn for users who do not use sha1dc
+
+if(NOT "${USE_SHA1}" STREQUAL "CollisionDetection")
+	list(APPEND WARNINGS "SHA1 support is set to ${USE_SHA1} which is not recommended - git's hash algorithm is sha1dc, it is *not* SHA1. Using SHA1 may leave you and your users susceptible to SHAttered-style attacks.")
+	set(WARNINGS ${WARNINGS} PARENT_SCOPE)
+endif()
