[PATCH v3 8/9] Makefile: allow specifying a SHA-1 for non-cryptographic uses

[Taylor Blau <me@ttaylorr.com>]

Introduce _FAST variants of the OPENSSL_SHA1, BLK_SHA1, and
APPLE_COMMON_CRYPTO_SHA1 compile-time knobs which indicate which SHA-1
implementation is to be used for non-cryptographic uses.

There are a couple of small implementation notes worth mentioning:

  - There is no way to select the collision detecting SHA-1 as the
    "fast" fallback, since the fast fallback is only for
    non-cryptographic uses, and is meant to be faster than our
    collision-detecting implementation.

  - There are no similar knobs for SHA-256, since no collision attacks
    are presently known and thus no collision-detecting implementations
    actually exist.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Makefile | 25 +++++++++++++++++++++++++
 hash.h   | 30 ++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/Makefile b/Makefile
index e298c8b55ec..d24f9088802 100644
--- a/Makefile
+++ b/Makefile
@@ -517,6 +517,10 @@ include shared.mak
 # Define APPLE_COMMON_CRYPTO_SHA1 to use Apple's CommonCrypto for
 # SHA-1.
 #
+# Define the same Makefile knobs as above, but suffixed with _FAST to
+# use the corresponding implementations for "fast" SHA-1 hashing for
+# non-cryptographic purposes.
+#
 # If don't enable any of the *_SHA1 settings in this section, Git will
 # default to its built-in sha1collisiondetection library, which is a
 # collision-detecting sha1 This is slower, but may detect attempted
@@ -1982,6 +1986,27 @@ endif
 endif
 endif
 
+ifdef OPENSSL_SHA1_FAST
+ifndef OPENSSL_SHA1
+	EXTLIBS += $(LIB_4_CRYPTO)
+	BASIC_CFLAGS += -DSHA1_OPENSSL_FAST
+endif
+else
+ifdef BLK_SHA1_FAST
+ifndef BLK_SHA1
+	LIB_OBJS += block-sha1/sha1.o
+	BASIC_CFLAGS += -DSHA1_BLK_FAST
+endif
+else
+ifdef APPLE_COMMON_CRYPTO_SHA1_FAST
+ifndef APPLE_COMMON_CRYPTO_SHA1
+	COMPAT_CFLAGS += -DCOMMON_DIGEST_FOR_OPENSSL
+	BASIC_CFLAGS += -DSHA1_APPLE_FAST
+endif
+endif
+endif
+endif
+
 ifdef OPENSSL_SHA256
 	EXTLIBS += $(LIB_4_CRYPTO)
 	BASIC_CFLAGS += -DSHA256_OPENSSL
diff --git a/hash.h b/hash.h
index 5e5b8205b58..6ea0d968017 100644
--- a/hash.h
+++ b/hash.h
@@ -15,6 +15,36 @@
 #include "block-sha1/sha1.h"
 #endif
 
+#if defined(SHA1_APPLE_FAST)
+#  include <CommonCrypto/CommonDigest.h>
+#  define platform_SHA_CTX_fast CC_SHA1_CTX
+#  define platform_SHA1_Init_fast CC_SHA1_Init
+#  define platform_SHA1_Update_fast CC_SHA1_Update
+#  define platform_SHA1_Final_fast CC_SHA1_Final
+#elif defined(SHA1_OPENSSL_FAST)
+#  include <openssl/sha.h>
+#  if defined(OPENSSL_API_LEVEL) && OPENSSL_API_LEVEL >= 3
+#    define SHA1_NEEDS_CLONE_HELPER_FAST
+#    include "sha1/openssl.h"
+#    define platform_SHA_CTX_fast openssl_SHA1_CTX
+#    define platform_SHA1_Init_fast openssl_SHA1_Init
+#    define platform_SHA1_Clone_fast openssl_SHA1_Clone
+#    define platform_SHA1_Update_fast openssl_SHA1_Update
+#    define platform_SHA1_Final_fast openssl_SHA1_Final
+#  else
+#    define platform_SHA_CTX_fast SHA_CTX
+#    define platform_SHA1_Init_fast SHA1_Init
+#    define platform_SHA1_Update_fast SHA1_Update
+#    define platform_SHA1_Final_fast SHA1_Final
+#  endif
+#elif defined(SHA1_BLK_FAST)
+#  include "block-sha1/sha1.h"
+#  define platform_SHA_CTX_fast blk_SHA_CTX
+#  define platform_SHA1_Init_fast blk_SHA1_Init
+#  define platform_SHA1_Update_fast blk_SHA1_Update
+#  define platform_SHA1_Final_fast blk_SHA1_Final
+#endif
+
 #if defined(SHA256_NETTLE)
 #include "sha256/nettle.h"
 #elif defined(SHA256_GCRYPT)
-- 
2.46.0.430.gca674632b70