From: Jeff King <peff@peff.net>
To: Theodore Ts'o <tytso@mit.edu>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
David Turner <dturner@twopensource.com>,
Stefan Beller <sbeller@google.com>,
"git@vger.kernel.org" <git@vger.kernel.org>
Subject: Re: Migrating away from SHA-1?
Date: Thu, 14 Apr 2016 22:13:27 -0400 [thread overview]
Message-ID: <20160415021327.GC22112@sigill.intra.peff.net> (raw)
In-Reply-To: <20160414224051.GD16656@thunk.org>
On Thu, Apr 14, 2016 at 06:40:51PM -0400, Theodore Ts'o wrote:
> Also, remember that while we can write programs that look for
> suspicious git objects that have stuff hidden after the null
> terminator (in fact, maybe that would be a good thing to add to git,
> hmmm?)[...]
Detecting the hidden bytes is underway elsewhere on the list. And while
I think it's a good idea to do so, I don't think it really introduces
a meaningful defense against collision attacks.
You can also hide bytes in arbitrary headers in a git object[1], and
they will not be shown by default. Adding the extra bytes at the end is
certainly easier if you're micro-optimizing the collision process[2],
but I don't think it changes the fundamental equation. It reduces the
work you do per-sha1 by a constant factor, but not the number of sha1s
you expect to compute.
-Peff
[1] Obviously neither "extra headers" nor "stuff after NUL" applies to
patches sent by email, where everything short of binary-diffs is
human-readable. So for the kernel, you're really talking about
attacking a lieutenant whose repo gets pulled. But there are plenty
of other projects that "git merge" from strangers.
[2] Somewhere in the list archive is my patch to find partial
collisions like "git commit --sha1=31337", and I did in fact use
that micro-optimization. That, along with multi-threading, made it
feasible to do 6-8 character prefixes, as I recall.
next prev parent reply other threads:[~2016-04-15 2:13 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-12 22:38 Migrating away from SHA-1? H. Peter Anvin
2016-04-12 23:00 ` Stefan Beller
2016-04-12 23:06 ` H. Peter Anvin
2016-04-12 23:15 ` Jeff King
2016-04-12 23:15 ` David Turner
2016-04-12 23:44 ` Jeff King
2016-04-14 1:53 ` Theodore Ts'o
2016-04-14 16:47 ` Joey Hess
2016-04-14 17:23 ` David Turner
2016-04-14 17:28 ` H. Peter Anvin
2016-04-14 22:40 ` Theodore Ts'o
2016-04-15 2:13 ` Jeff King [this message]
2016-04-15 2:18 ` Junio C Hamano
2016-04-15 2:22 ` Jeff King
2016-04-12 23:42 ` Jeff King
2016-04-13 1:03 ` Junio C Hamano
2016-04-13 1:36 ` Jeff King
2016-04-13 1:38 ` H. Peter Anvin
2016-04-13 1:51 ` Duy Nguyen
2016-04-13 1:58 ` H. Peter Anvin
2016-04-15 1:50 ` brian m. carlson
-- strict thread matches above, loose matches on Subject: below --
2016-06-18 2:10 Leo Gaspard
2016-06-18 3:30 ` Eric Wong
2016-06-24 18:17 ` brian m. carlson
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