Subject: Re: Exploiting SHA1's "XOR weakness" allows for faster hash calculation

From: Theodore Ts'o <[email-masked]>
Date: Wed, 5 Dec 2012 12:20:11 -0500

On Wed, Dec 05, 2012 at 10:19:43AM +0100, Sebastian Schuberth wrote:
> 
> to say it in advance: I do not want to trigger any bogus security
> discussion here. Instead, I believe the findings from [1] allow for
> an up to 20% faster SHA1 calculation, if my brief reading of the
> presentation is correct. Any opinions on integration this
> optimization into Git?
> 
> [1] https://hashcat.net/p12/js-sha1exp_169.pdf

It's only useful if you are trying to do brute-force password
cracking, where the password is being hashed in a very specific way.
(If for example the password was replicated N times in the input
buffer for SHA-1, instead of keeping the padding constant in the rest
of theinput buffer, this particular optimization would't apply.)

In any case, it's not at all applicable for general purpose checksum
calculations, and hence wouldn't apply to git.

Regards,

						- Ted