From: David Lang <david@lang.hm>
Date: Thu, 23 Feb 2017 17:00:20 GMT
Subject: Re: SHA1 collisions found
Message-ID: <nycvar.QRO.7.75.62.1702230857420.6590@qynat-yncgbc>
In-Reply-To: <20170223164306.spg2avxzukkggrpb@kitenet.net>

On Thu, 23 Feb 2017, Joey Hess wrote:

> https://shattered.io/static/shattered.pdf
> https://freedom-to-tinker.com/2017/02/23/rip-sha-1/
>
> IIRC someone has been working on parameterizing git's SHA1 assumptions
> so a repository could eventually use a more secure hash. How far has
> that gotten? There are still many "40" constants in git.git HEAD.
>
> In the meantime, git commit -S, and checks that commits are signed,
> seems like the only way to mitigate against attacks such as
> the ones described in the threads at
> https://joeyh.name/blog/sha-1/ and
> https://joeyh.name/blog/entry/size_of_the_git_sha1_collision_attack_surface/
>
> Since we now have collisions in valid PDF files, collisions in valid git
> commit and tree objects are probably able to be constructed.

keep in mind that there is a huge difference between

creating a collision between two documents you create, both of which contain a 
huge amount of arbitrary binary data that can be changed at will without 
affecting the results

and

creating a collision betwen an existing document that someone else created and a 
new document that is also valid C code without huge amounts of binary in it.

So, it's not time to panic, but it is one more push to make the changes to 
support something else.

David Lang