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, 45 insertions, 0 deletions

diff --git a/sha1-file.c b/sha1-file.c
index 9aadaf0c8d..66ba3dadb9 100644
--- a/sha1-file.c
+++ b/sha1-file.c
@@ -40,10 +40,20 @@
 #define EMPTY_TREE_SHA1_BIN_LITERAL \
 	 "\x4b\x82\x5d\xc6\x42\xcb\x6e\xb9\xa0\x60" \
 	 "\xe5\x4b\xf8\xd6\x92\x88\xfb\xee\x49\x04"
+#define EMPTY_TREE_SHA256_BIN_LITERAL \
+	"\x6e\xf1\x9b\x41\x22\x5c\x53\x69\xf1\xc1" \
+	"\x04\xd4\x5d\x8d\x85\xef\xa9\xb0\x57\xb5" \
+	"\x3b\x14\xb4\xb9\xb9\x39\xdd\x74\xde\xcc" \
+	"\x53\x21"
 
 #define EMPTY_BLOB_SHA1_BIN_LITERAL \
 	"\xe6\x9d\xe2\x9b\xb2\xd1\xd6\x43\x4b\x8b" \
 	"\x29\xae\x77\x5a\xd8\xc2\xe4\x8c\x53\x91"
+#define EMPTY_BLOB_SHA256_BIN_LITERAL \
+	"\x47\x3a\x0f\x4c\x3b\xe8\xa9\x36\x81\xa2" \
+	"\x67\xe3\xb1\xe9\xa7\xdc\xda\x11\x85\x43" \
+	"\x6f\xe1\x41\xf7\x74\x91\x20\xa3\x03\x72" \
+	"\x18\x13"
 
 const unsigned char null_sha1[GIT_MAX_RAWSZ];
 const struct object_id null_oid;
@@ -53,6 +63,12 @@ static const struct object_id empty_tree_oid = {
 static const struct object_id empty_blob_oid = {
 	EMPTY_BLOB_SHA1_BIN_LITERAL
 };
+static const struct object_id empty_tree_oid_sha256 = {
+	EMPTY_TREE_SHA256_BIN_LITERAL
+};
+static const struct object_id empty_blob_oid_sha256 = {
+	EMPTY_BLOB_SHA256_BIN_LITERAL
+};
 
 static void git_hash_sha1_init(git_hash_ctx *ctx)
 {
@@ -69,6 +85,22 @@ static void git_hash_sha1_final(unsigned char *hash, git_hash_ctx *ctx)
 	git_SHA1_Final(hash, &ctx->sha1);
 }
 
+
+static void git_hash_sha256_init(git_hash_ctx *ctx)
+{
+	git_SHA256_Init(&ctx->sha256);
+}
+
+static void git_hash_sha256_update(git_hash_ctx *ctx, const void *data, size_t len)
+{
+	git_SHA256_Update(&ctx->sha256, data, len);
+}
+
+static void git_hash_sha256_final(unsigned char *hash, git_hash_ctx *ctx)
+{
+	git_SHA256_Final(hash, &ctx->sha256);
+}
+
 static void git_hash_unknown_init(git_hash_ctx *ctx)
 {
 	BUG("trying to init unknown hash");
@@ -110,6 +142,19 @@ const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
 		&empty_tree_oid,
 		&empty_blob_oid,
 	},
+	{
+		"sha256",
+		/* "s256", big-endian */
+		0x73323536,
+		GIT_SHA256_RAWSZ,
+		GIT_SHA256_HEXSZ,
+		GIT_SHA256_BLKSZ,
+		git_hash_sha256_init,
+		git_hash_sha256_update,
+		git_hash_sha256_final,
+		&empty_tree_oid_sha256,
+		&empty_blob_oid_sha256,
+	}
 };
 
 const char *empty_tree_oid_hex(void)