Re: SHA1 collisions found
From: Thomas Braun <hidden>
Date: 2017-02-26 18:57:28
Am 25.02.2017 um 00:06 schrieb Jeff King:
quoted hunk ↗ jump to hunk
So we don't actually know how Git would behave in the face of a SHA-1 collision. It would be pretty easy to simulate it with something like: ---diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c index 22b125cf8..1be5b5ba3 100644 --- a/block-sha1/sha1.c +++ b/block-sha1/sha1.c@@ -231,6 +231,16 @@ void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *data, unsigned long len) memcpy(ctx->W, data, len); } +/* sha1 of blobs containing "foo\n" and "bar\n" */ +static const unsigned char foo_sha1[] = { + 0x25, 0x7c, 0xc5, 0x64, 0x2c, 0xb1, 0xa0, 0x54, 0xf0, 0x8c, + 0xc8, 0x3f, 0x2d, 0x94, 0x3e, 0x56, 0xfd, 0x3e, 0xbe, 0x99 +}; +static const unsigned char bar_sha1[] = { + 0x57, 0x16, 0xca, 0x59, 0x87, 0xcb, 0xf9, 0x7d, 0x6b, 0xb5, + 0x49, 0x20, 0xbe, 0xa6, 0xad, 0xde, 0x24, 0x2d, 0x87, 0xe6 +}; + void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx) { static const unsigned char pad[64] = { 0x80 };@@ -248,4 +258,8 @@ void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx) /* Output hash */ for (i = 0; i < 5; i++) put_be32(hashout + i * 4, ctx->H[i]); + + /* pretend "foo" and "bar" collide */ + if (!memcmp(hashout, bar_sha1, 20)) + memcpy(hashout, foo_sha1, 20); }
While reading about the subject I came across [1]. The author reduced the hash size to 4bits and then played around with git. Diff taken from the posting (not my code)
--- git-2.7.0~rc0+next.20151210.orig/block-sha1/sha1.c
+++ git-2.7.0~rc0+next.20151210/block-sha1/sha1.c@@ -246,6 +246,8 @@ void blk_SHA1_Final(unsigned char hashou blk_SHA1_Update(ctx, padlen, 8); /* Output hash */ - for (i = 0; i < 5; i++) - put_be32(hashout + i * 4, ctx->H[i]); + for (i = 0; i < 1; i++) + put_be32(hashout + i * 4, (ctx->H[i] & 0xf000000)); + for (i = 1; i < 5; i++) + put_be32(hashout + i * 4, 0); }
From a noob git-dev perspective this sounds more flexibel. [1]: http://stackoverflow.com/a/34599081