From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-4.1 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD shortcircuit=no autolearn=ham autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id AAC00201A9 for ; Fri, 24 Feb 2017 23:06:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751334AbdBXXGI (ORCPT ); Fri, 24 Feb 2017 18:06:08 -0500 Received: from cloud.peff.net ([104.130.231.41]:33700 "EHLO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751320AbdBXXGH (ORCPT ); Fri, 24 Feb 2017 18:06:07 -0500 Received: (qmail 20144 invoked by uid 109); 24 Feb 2017 23:06:07 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.84) with SMTP; Fri, 24 Feb 2017 23:06:07 +0000 Received: (qmail 15870 invoked by uid 111); 24 Feb 2017 23:06:11 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.84) with SMTP; Fri, 24 Feb 2017 18:06:11 -0500 Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Fri, 24 Feb 2017 18:06:05 -0500 Date: Fri, 24 Feb 2017 18:06:05 -0500 From: Jeff King To: Jakub =?utf-8?B?TmFyxJlic2tp?= Cc: Joey Hess , git@vger.kernel.org Subject: Re: SHA1 collisions found Message-ID: <20170224230604.nt37uw5y3uehukfd@sigill.intra.peff.net> References: <20170223164306.spg2avxzukkggrpb@kitenet.net> <9cedbfa5-4095-15d8-639c-0e3b9b98d6b9@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <9cedbfa5-4095-15d8-639c-0e3b9b98d6b9@gmail.com> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Fri, Feb 24, 2017 at 11:47:46PM +0100, Jakub Narębski wrote: > I have just read on ArsTechnica[1] that while Git repository could be > corrupted (though this would require attackers to spend great amount > of resources creating their own collision, while as said elsewhere > in this thread allegedly easy to detect), putting two proof-of-concept > different PDFs with same size and SHA-1 actually *breaks* Subversion. > Repository can become corrupt, and stop accepting new commits. > > From what I understand people tried this, and Git doesn't exhibit > such problem. I wonder what assumptions SVN made that were broken... To be clear, nobody has generated a sha1 collision in Git yet, and you cannot blindly use the shattered PDFs to do so. Git's notion of the SHA-1 of an object include the header, so somebody would have to do a shattered-level collision search for something that starts with the correct "blob 1234\0" header. 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); }