Add a base implementation of SHA-256 support

SHA-1 is weak and we need to transition to a new hash function.  For
some time, we have referred to this new function as NewHash.  Recently,
we decided to pick SHA-256 as NewHash.  The reasons behind the choice of
SHA-256 are outlined in the thread starting at [1] and in the commit
history for the hash function transition document.

Add a basic implementation of SHA-256 based off libtomcrypt, which is in
the public domain.  Optimize it and restructure it to meet our coding
standards.  Pull in the update and final functions from the SHA-1 block
implementation, as we know these function correctly with all compilers.
This implementation is slower than SHA-1, but more performant
implementations will be introduced in future commits.

Wire up SHA-256 in the list of hash algorithms, and add a test that the
algorithm works correctly.

Note that with this patch, it is still not possible to switch to using
SHA-256 in Git.  Additional patches are needed to prepare the code to
handle a larger hash algorithm and further test fixes are needed.

[1] https://public-inbox.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/hash.h b/hash.h
index 1bcf7ab6fd..a9bc624020 100644
--- a/hash.h
+++ b/hash.h
@@ -15,6 +15,8 @@
 #include "block-sha1/sha1.h"
 #endif
 
+#include "sha256/block/sha256.h"
+
 #ifndef platform_SHA_CTX
 /*
  * platform's underlying implementation of SHA-1; could be OpenSSL,
@@ -34,6 +36,18 @@
 #define git_SHA1_Update		platform_SHA1_Update
 #define git_SHA1_Final		platform_SHA1_Final
 
+#ifndef platform_SHA256_CTX
+#define platform_SHA256_CTX	SHA256_CTX
+#define platform_SHA256_Init	SHA256_Init
+#define platform_SHA256_Update	SHA256_Update
+#define platform_SHA256_Final	SHA256_Final
+#endif
+
+#define git_SHA256_CTX		platform_SHA256_CTX
+#define git_SHA256_Init		platform_SHA256_Init
+#define git_SHA256_Update	platform_SHA256_Update
+#define git_SHA256_Final	platform_SHA256_Final
+
 #ifdef SHA1_MAX_BLOCK_SIZE
 #include "compat/sha1-chunked.h"
 #undef git_SHA1_Update
@@ -52,12 +66,15 @@
 #define GIT_HASH_UNKNOWN 0
 /* SHA-1 */
 #define GIT_HASH_SHA1 1
+/* SHA-256  */
+#define GIT_HASH_SHA256 2
 /* Number of algorithms supported (including unknown). */
-#define GIT_HASH_NALGOS (GIT_HASH_SHA1 + 1)
+#define GIT_HASH_NALGOS (GIT_HASH_SHA256 + 1)
 
 /* A suitably aligned type for stack allocations of hash contexts. */
 union git_hash_ctx {
 	git_SHA_CTX sha1;
+	git_SHA256_CTX sha256;
 };
 typedef union git_hash_ctx git_hash_ctx;