Subject: [PATCH v3 2/2] help: include unsafe SHA-1 build info in version

From: Justin Tobler <[email-masked]>
Date: Thu, 3 Apr 2025 09:05:29 -0500

In 06c92dafb8 (Makefile: allow specifying a SHA-1 for non-cryptographic
uses, 2024-09-26), support for unsafe SHA-1 is added. Add the unsafe
SHA-1 build info to `git version --build-info` and update corresponding
documentation.

Signed-off-by: Justin Tobler <[email-masked]>
---
 Documentation/git-version.adoc | 4 +++-
 hash.h                         | 3 +++
 help.c                         | 4 ++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-version.adoc b/Documentation/git-version.adoc
index 913ebf147d..9462043a14 100644
--- a/Documentation/git-version.adoc
+++ b/Documentation/git-version.adoc
@@ -27,7 +27,9 @@ The libraries used to implement the SHA-1 and SHA-256 algorithms are displayed
 in the form `SHA-1: <option>` and `SHA-256: <option>`. Note that the SHA-1
 options `SHA1_APPLE`, `SHA1_OPENSSL`, and `SHA1_BLK` do not use a collision
 detection algorithm and thus may be vulnerable to known SHA-1 collision
-attacks.
+attacks. When a faster SHA-1 implementation without collision detection is used
+for only non-cryptographic purposes, the algorithm is displayed in the form
+`non-collision-detecting-SHA-1: <option>`.
 
 GIT
 ---
diff --git a/hash.h b/hash.h
index 51cd0ec7b6..72334d3506 100644
--- a/hash.h
+++ b/hash.h
@@ -20,12 +20,14 @@
 #endif
 
 #if defined(SHA1_APPLE_UNSAFE)
+#  define SHA1_UNSAFE_BACKEND "SHA1_APPLE_UNSAFE"
 #  include <CommonCrypto/CommonDigest.h>
 #  define platform_SHA_CTX_unsafe CC_SHA1_CTX
 #  define platform_SHA1_Init_unsafe CC_SHA1_Init
 #  define platform_SHA1_Update_unsafe CC_SHA1_Update
 #  define platform_SHA1_Final_unsafe CC_SHA1_Final
 #elif defined(SHA1_OPENSSL_UNSAFE)
+#  define SHA1_UNSAFE_BACKEND "SHA1_OPENSSL_UNSAFE"
 #  include <openssl/sha.h>
 #  if defined(OPENSSL_API_LEVEL) && OPENSSL_API_LEVEL >= 3
 #    define SHA1_NEEDS_CLONE_HELPER_UNSAFE
@@ -42,6 +44,7 @@
 #    define platform_SHA1_Final_unsafe SHA1_Final
 #  endif
 #elif defined(SHA1_BLK_UNSAFE)
+#  define SHA1_UNSAFE_BACKEND "SHA1_BLK_UNSAFE"
 #  include "block-sha1/sha1.h"
 #  define platform_SHA_CTX_unsafe blk_SHA_CTX
 #  define platform_SHA1_Init_unsafe blk_SHA1_Init
diff --git a/help.c b/help.c
index 991a9525db..6ef90838f1 100644
--- a/help.c
+++ b/help.c
@@ -805,6 +805,10 @@ void get_version_info(struct strbuf *buf, int show_build_options)
 		strbuf_addf(buf, "zlib: %s\n", ZLIB_VERSION);
 #endif
 		strbuf_addf(buf, "SHA-1: %s\n", SHA1_BACKEND);
+#if defined SHA1_UNSAFE_BACKEND
+		strbuf_addf(buf, "non-collision-detecting-SHA-1: %s\n",
+			    SHA1_UNSAFE_BACKEND);
+#endif
 		strbuf_addf(buf, "SHA-256: %s\n", SHA256_BACKEND);
 	}
 }
-- 
2.49.0