On Sun, Feb 26, 2017 at 07:57:19PM +0100, Thomas Braun wrote:
quoted hunk ↗ jump to hunk
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);
}
Yeah, that is a lot more flexible for experimenting. Though I'd think
you'd probably want more than 4 bits just to avoid accidental
collisions. Something like 24 bits gives you some breathing space (you'd
expect a random collision after 4096 objects), but it's still easy to
do a preimage attack if you need to.
-Peff