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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Joey Hess <id@joeyh.name>
Cc: Git Mailing List <git@vger.kernel.org>
Subject: Re: SHA1 collisions found
Date: Thu, 23 Feb 2017 10:40:48 -0800	[thread overview]
Message-ID: <CA+55aFxckeEW1ePcebrgG4iN4Lp62A2vU6tA=xnSDC_BnKQiCQ@mail.gmail.com> (raw)
In-Reply-To: <20170223182147.hbsyxsmyijgkqu75@kitenet.net>

On Thu, Feb 23, 2017 at 10:21 AM, Joey Hess <id@joeyh.name> wrote:
> Linus Torvalds wrote:
>> What you describe pretty much already requires a pre-image attack,
>> which the new attack is _not_.
>>
>> It's not clear that the "good" object can be anything sane.
>
> Generate a regular commit object; use the entire commit object + NUL as the
> chosen prefix, and use the identical-prefix collision attack to generate
> the colliding good/bad objects.

So I agree with you that we need to make git check for the opaque
data. I think I was the one who brought that whole argument up.

But even then, what you describe doesn't work. What you describe just
replaces the opaque data - that git doesn't actually *use*, and that
nobody sees - with another piece of opaque data.

You also need to make the non-opaque data of the bad object be
something that actually encodes valid git data with interesting hashes
in it (for the parent/tree/whatever pointers).

So you don't have just that "chosen prefix". You actually need to also
fill in some very specific piece of data *in* the attack parts itself.
And you need to do this in the exact same size (because that's part of
the prefix), etc etc.

So I think it's challenging.

... and then we can discover it trivially.

Ok, so "git fsck" right now takes a couple of minutes for me and I
don't actually run it very often (I used to run it religiously back in
the days), but afaik kernel.org actually runs it nightly. So it's
pretty much "trivially discoverable" - imagine spending thousands of
CPU-hours and lots of social capital to get an attack in, and then the
next night the kernel.org fsck complains about the strange commit you
added?

                  Linus

  parent reply	other threads:[~2017-02-23 18:40 UTC|newest]

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-02-23 16:43 SHA1 collisions found Joey Hess
2017-02-23 17:00 ` David Lang
2017-02-23 17:02 ` Junio C Hamano
2017-02-23 17:12   ` David Lang
2017-02-23 20:49     ` Jakub Narębski
2017-02-23 20:57       ` Jeff King
2017-02-23 17:18   ` Junio C Hamano
2017-02-23 17:35   ` Joey Hess
2017-02-23 17:52     ` Linus Torvalds
2017-02-23 18:21       ` Joey Hess
2017-02-23 18:31         ` Joey Hess
2017-02-23 19:13           ` Morten Welinder
2017-02-24 15:52             ` Geert Uytterhoeven
2017-02-23 18:40         ` Linus Torvalds [this message]
2017-02-23 18:46           ` Jeff King
2017-02-23 19:09             ` Linus Torvalds
2017-02-23 19:32               ` Jeff King
2017-02-23 19:47                 ` Linus Torvalds
2017-02-23 19:57                   ` Jeff King
     [not found]                     ` <alpine.LFD.2.20.1702231428540.30435@i7.lan>
2017-02-23 22:43                       ` Jeff King
2017-02-23 22:50                         ` Linus Torvalds
2017-02-23 23:05                         ` Jeff King
2017-02-23 23:05                           ` [PATCH 1/3] add collision-detecting sha1 implementation Jeff King
2017-02-23 23:15                             ` Stefan Beller
2017-02-24  0:01                               ` Jeff King
2017-02-24  0:12                                 ` Linus Torvalds
2017-02-24  0:16                                   ` Jeff King
2017-02-23 23:05                           ` [PATCH 2/3] sha1dc: adjust header includes for git Jeff King
2017-02-23 23:06                           ` [PATCH 3/3] Makefile: add USE_SHA1DC knob Jeff King
2017-02-24 18:36                             ` HW42
2017-02-24 18:57                               ` Jeff King
2017-02-23 23:14                           ` SHA1 collisions found Linus Torvalds
2017-02-28 18:41                           ` Junio C Hamano
2017-02-28 19:07                             ` Junio C Hamano
2017-02-28 19:20                               ` Jeff King
2017-03-01  8:57                                 ` Dan Shumow
2017-02-28 19:34                               ` Linus Torvalds
2017-02-28 19:52                                 ` Shawn Pearce
2017-02-28 22:56                                   ` Linus Torvalds
2017-02-28 21:22                                 ` Dan Shumow
2017-02-28 22:50                                   ` Marc Stevens
2017-02-28 23:11                                     ` Linus Torvalds
2017-03-01 19:05                                       ` Jeff King
2017-02-23 20:47               ` Øyvind A. Holm
2017-02-23 20:46             ` Joey Hess
2017-02-23 18:42         ` Jeff King
2017-02-23 17:52     ` David Lang
2017-02-23 19:20   ` David Lang
2017-02-23 17:19 ` Linus Torvalds
2017-02-23 17:29   ` Linus Torvalds
2017-02-23 18:10   ` Joey Hess
2017-02-23 18:29     ` Linus Torvalds
2017-02-23 18:38     ` Junio C Hamano
2017-02-24  9:42 ` Duy Nguyen
2017-02-25 19:04   ` brian m. carlson
2017-02-27 13:29     ` René Scharfe
2017-02-28 13:25       ` brian m. carlson
2017-02-24 15:13 ` Ian Jackson
2017-02-24 17:04   ` ankostis
2017-02-24 17:23   ` Jason Cooper
2017-02-25 23:22     ` ankostis
2017-02-24 17:32   ` Junio C Hamano
2017-02-24 17:45     ` David Lang
2017-02-24 18:14       ` Junio C Hamano
2017-02-24 18:58         ` Stefan Beller
2017-02-24 19:20           ` Junio C Hamano
2017-02-24 20:05             ` ankostis
2017-02-24 20:32               ` Junio C Hamano
2017-02-25  0:31                 ` ankostis
2017-02-26  0:16                   ` Jason Cooper
2017-02-26 17:38                     ` brian m. carlson
2017-02-26 19:11                       ` Linus Torvalds
2017-02-26 21:38                         ` Ævar Arnfjörð Bjarmason
2017-02-26 21:52                           ` Jeff King
2017-02-27 13:00                             ` Transition plan for git to move to a new hash function Ian Jackson
2017-02-27 14:37                               ` Why BLAKE2? Markus Trippelsdorf
2017-02-27 15:42                                 ` Ian Jackson
2017-02-27 19:26                               ` Transition plan for git to move to a new hash function Tony Finch
2017-02-28 21:47                               ` brian m. carlson
2017-03-02 18:13                                 ` Ian Jackson
2017-03-04 22:49                                   ` brian m. carlson
2017-03-05 13:45                                     ` Ian Jackson
2017-03-05 23:45                                       ` brian m. carlson
2017-02-24 20:05             ` SHA1 collisions found Junio C Hamano
2017-02-24 20:33           ` Philip Oakley
2017-02-24 23:39     ` Jeff King
2017-02-25  0:39       ` Linus Torvalds
2017-02-25  0:54         ` Linus Torvalds
2017-02-25  1:16         ` Jeff King
2017-02-26 18:55           ` Junio C Hamano
2017-02-25  6:10         ` Junio C Hamano
2017-02-26  1:13           ` Jason Cooper
2017-02-26  5:18             ` Jeff King
2017-02-26 18:30               ` brian m. carlson
2017-03-02 21:46               ` Brandon Williams
2017-03-03 11:13                 ` Jeff King
2017-03-03 14:54                   ` Ian Jackson
2017-03-03 22:18                     ` Jeff King
2017-03-02 19:55         ` Linus Torvalds
2017-03-02 20:43           ` Junio C Hamano
2017-03-02 21:21             ` Linus Torvalds
2017-03-02 21:54               ` Joey Hess
2017-03-02 22:27                 ` Linus Torvalds
2017-03-03  1:50                   ` Mike Hommey
2017-03-03  2:19                     ` Linus Torvalds
2017-03-03 11:04           ` Jeff King
2017-03-03 21:47           ` Stefan Beller
2017-02-25  1:00       ` David Lang
2017-02-25  1:15         ` Stefan Beller
2017-02-25  1:21         ` Jeff King
2017-02-25  1:39           ` David Lang
2017-02-25  1:47             ` Jeff King
2017-02-25  1:56               ` David Lang
2017-02-25  2:28             ` Jacob Keller
2017-02-25  2:26           ` Jacob Keller
2017-02-25  5:39             ` grarpamp
2017-02-24 23:43     ` Ian Jackson
2017-02-25  0:06       ` Ian Jackson
2017-02-25 18:50     ` brian m. carlson
2017-02-25 19:26       ` Jeff King
2017-02-25 22:09         ` Mike Hommey
2017-02-26 17:38           ` brian m. carlson
2017-02-24 22:47 ` Jakub Narębski
2017-02-24 22:53   ` Santiago Torres
2017-02-24 23:05     ` Jakub Narębski
2017-02-24 23:24       ` Øyvind A. Holm
2017-02-24 23:06   ` Jeff King
2017-02-24 23:35     ` Jakub Narębski
2017-02-25 22:35     ` Lars Schneider
2017-02-26  0:46       ` Jeff King
2017-02-26 18:22         ` Junio C Hamano
2017-02-26 18:57     ` Thomas Braun
2017-02-26 21:30       ` Jeff King
2017-02-27  9:57         ` Geert Uytterhoeven
2017-02-27 10:43           ` Jeff King
2017-02-27 12:39             ` Morten Welinder

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