sha256: add an SHA-256 implementation using libgcrypt

Generally, one gets better performance out of cryptographic routines
written in assembly than C, and this is also true for SHA-256.  In
addition, most Linux distributions cannot distribute Git linked against
OpenSSL for licensing reasons.

Most systems with GnuPG will also have libgcrypt, since it is a
dependency of GnuPG.  libgcrypt is also faster than the SHA1DC
implementation for messages of a few KiB and larger.

For comparison, on a Core i7-6600U, this implementation processes 16 KiB
chunks at 355 MiB/s while SHA1DC processes equivalent chunks at 337
MiB/s.

In addition, libgcrypt is licensed under the LGPL 2.1, which is
compatible with the GPL.  Add an implementation of SHA-256 that uses
libgcrypt.

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

Makefile

@@ -179,6 +179,10 @@ all::
 # in one call to the platform's SHA1_Update(). e.g. APPLE_COMMON_CRYPTO
 # wants 'SHA1_MAX_BLOCK_SIZE=1024L*1024L*1024L' defined.
 #
+# Define BLK_SHA256 to use the built-in SHA-256 routines.
+#
+# Define GCRYPT_SHA256 to use the SHA-256 routines in libgcrypt.
+#
 # Define NEEDS_CRYPTO_WITH_SSL if you need -lcrypto when using -lssl (Darwin).
 #
 # Define NEEDS_SSL_WITH_CRYPTO if you need -lssl when using -lcrypto (Darwin).
@@ -1634,8 +1638,13 @@ endif
 endif
 endif
 
-LIB_OBJS += sha256/block/sha256.o
-BASIC_CFLAGS += -DSHA256_BLK
+ifdef GCRYPT_SHA256
+	BASIC_CFLAGS += -DSHA256_GCRYPT
+	EXTLIBS += -lgcrypt
+else
+	LIB_OBJS += sha256/block/sha256.o
+	BASIC_CFLAGS += -DSHA256_BLK
+endif
 
 ifdef SHA1_MAX_BLOCK_SIZE
 	LIB_OBJS += compat/sha1-chunked.o

hash.h

@@ -15,7 +15,11 @@
 #include "block-sha1/sha1.h"
 #endif
 
+#if defined(SHA256_GCRYPT)
+#include "sha256/gcrypt.h"
+#else
 #include "sha256/block/sha256.h"
+#endif
 
 #ifndef platform_SHA_CTX
 /*

sha256/gcrypt.h

@@ -0,0 +1,30 @@
+#ifndef SHA256_GCRYPT_H
+#define SHA256_GCRYPT_H
+
+#include <gcrypt.h>
+
+#define SHA256_DIGEST_SIZE 32
+
+typedef gcry_md_hd_t gcrypt_SHA256_CTX;
+
+inline void gcrypt_SHA256_Init(gcrypt_SHA256_CTX *ctx)
+{
+	gcry_md_open(ctx, GCRY_MD_SHA256, 0);
+}
+
+inline void gcrypt_SHA256_Update(gcrypt_SHA256_CTX *ctx, const void *data, size_t len)
+{
+	gcry_md_write(*ctx, data, len);
+}
+
+inline void gcrypt_SHA256_Final(unsigned char *digest, gcrypt_SHA256_CTX *ctx)
+{
+	memcpy(digest, gcry_md_read(*ctx, GCRY_MD_SHA256), SHA256_DIGEST_SIZE);
+}
+
+#define platform_SHA256_CTX gcrypt_SHA256_CTX
+#define platform_SHA256_Init gcrypt_SHA256_Init
+#define platform_SHA256_Update gcrypt_SHA256_Update
+#define platform_SHA256_Final gcrypt_SHA256_Final
+
+#endif