On Thu, Jun 15, 2017 at 09:01:45AM -0400, Jeff King wrote: > On Thu, Jun 15, 2017 at 08:05:18PM +0900, Mike Hommey wrote: > > > On Thu, Jun 15, 2017 at 12:30:46PM +0200, Johannes Schindelin wrote: > > > Footnote *1*: SHA-256, as all hash functions whose output is essentially > > > the entire internal state, are susceptible to a so-called "length > > > extension attack", where the hash of a secret+message can be used to > > > generate the hash of secret+message+piggyback without knowing the secret. > > > This is not the case for Git: only visible data are hashed. The type of > > > attacks Git has to worry about is very different from the length extension > > > attacks, and it is highly unlikely that that weakness of SHA-256 leads to, > > > say, a collision attack. > > > > What do the experts think or SHA512/256, which completely removes the > > concerns over length extension attack? (which I'd argue is better than > > sweeping them under the carpet) > > I don't think it's sweeping them under the carpet. Git does not use the > hash as a MAC, so length extension attacks aren't a thing (and even if > we later wanted to use the same algorithm as a MAC, the HMAC > construction is a well-studied technique for dealing with it).
AIUI, length extension does make brute force collision attacks (which, really Shattered was) cheaper by allowing one to create the collision with a small message and extend it later. This might not be a credible thread against git, but if we go by that standard, post-shattered Sha-1 is still fine for git. As a matter of fact, MD5 would also be fine: there is still, to this day, no preimage attack against them. Mike
