Re: Migrating away from SHA-1?

From: Joey Hess <id@joeyh.name>
Date: Apr 14, 2016, 16:47 UTC
Message-ID: <20160414164751.GA3255@kitenet.net>
In-Reply-To: <20160414015324.GA16656@thunk.org>

Theodore Ts'o wrote:

> OK, so how does this map to git?  First of all, from a collision
> perspective, the two blobs have to map into valid C code

Git provides other places to hide the colliding blobs; the best seems to be as an added header in the commit object, or as trailing data after a \0 in the commit message. git is very good at hiding such potentially colliding data from the user, as https://github.com/joeyh/supercollider demonstrates.

commit 24f30db5790b209fa412ce81c5ef2bf8af5fd4d7
Author: Joey Hess <joey@kitenet.net>
Date:   Fri Sep 9 11:49:21 2011 -0400

    an innocent commit
    
    If this were a sha1 colliding attack, there would be some sort of binary
    garbage below. Which there isn't. So this can be safely merged.

joey@darkstar:~/tmp/supercollider>git cat-file -p 24f30db5790b209fa412ce81c5ef2bf8af5fd4d7 tree 735a7633237c07b398856005de3bc9ea00446747 author Joey Hess <joey@kitenet.net> 1315583361 -0400 committer Joey Hess <joey@kitenet.net> 1315583361 -0400

an innocent commit

If this were a sha1 colliding attack, there would be some sort of binary garbage below. Which there isn't. So this can be safely merged.

??b???[?i??ͯ?t?2??????os?<????h?+,M?mY?e?EW?iv$???J??U}n~???L??????f???ě??3>?Q??H?޸*zl?RA˂q?E?E7???\?m???U?>MU GY?d)?ȼ??'g?~D??ɯhQ????/"E??X?m???^͸??S?D??;w6(?`??>?縘?AѲ?*!??@v????>?8??2?!??=*?J ???ynH???c?w?\??K7???N?6?????A5?FM?wZ?~?pKY?R???s7??(?ƶ?_"??m%????1a??ʀ??K[ t????!A0?ΈfT.?T?w?򁛵ƌ?р???aco?V/2??nَ? ?}?6?_?z?{

(The other possibility would be to hide the colliding blob in the tree object, but that seems unlikely.)

-- 
see shy jo