Re: Questions about the hash function transition

[Jonathan Nieder <jrnieder@gmail.com>]

Ævar Arnfjörð Bjarmason wrote:

>> Objective
>> ---------
>> Migrate Git from SHA-1 to a stronger hash function.
>
> Should way say "Migrate Git from SHA-1 to SHA-256" here instead?
>
> Maybe it's overly specific, i.e. really we're also describnig how /any/
> hash function transition might happen, but having just read this now
> from start to finish it takes us a really long time to mention (and at
> first, only offhand) that SHA-256 is the new hash.

I answered this question in my other reply, but my answer missed the
point.

I think it would be fine for this to say "Migrate Git from SHA-1 to a
stronger hash function (SHA-256)".  More importantly, I think the
Background section should say something about SHA-256 --- e.g. how about
replacing the sentence

  SHA-1 still possesses the other properties such as fast object
  lookup and safe error checking, but other hash functions are equally
  suitable that are believed to be cryptographically secure.

with something about SHA-256?

Rereading the background section, I see some other bits that could be
clarified, too.  It has a run-on sentence:

  Thus Git has in effect already migrated to a new hash that isn't
  SHA-1 and doesn't share its vulnerabilities, its new hash function
  just happens to produce exactly the same output for all known
  inputs, except two PDFs published by the SHAttered researchers, and
  the new implementation (written by those researchers) claims to
  detect future cryptanalytic collision attacks.

The "," after vulnerabilities should be a period, ending the sentence.
My understanding is that sha1collisiondetection's safe-hash is meant
to protect against known attacks and that the code is meant to be
adaptable for future attacks of the same kind (by updating the list of
disturbance vectors), but it doesn't claim to guard against future
novel cryptanalysis methods that haven't been published yet.

Thanks,
Jonathan