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.
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