Re: SHA1 collisions found

On Fri, Feb 24, 2017 at 09:32:13AM -0800, Junio C Hamano wrote:

Ian Jackson [off-list ref] writes:

quoted

I have been thinking about how to do a transition from SHA1 to another
hash function.

Good.  I think many of us have also been, too, not necessarily just
in the past few days in response to shattered, but over the last 10
years, yet without coming to a consensus design ;-)

quoted

I have concluded that:

 * We can should avoid expecting everyone to rewrite all their
   history.

Yes.

There are security implications for old objects if we mix hashes, but I
suppose people who want better security will just rewrite history
anyway.

As long as the reader can tell from the format of object names
stored in the "new object format" object from what era is being
referred to in some way [*1*], we can name new objects with only new
hash, I would think.  "new refers only to new" that stratifies
objects into older and newer may make things simpler, but I am not
convinced yet that it would give our users a smooth enough
transition path (but I am open to be educated and pursuaded the
other way).

I would simply use multihash[0] for this purpose.  New-style objects
serialize data in multihash format, so it's immediately obvious what
hash we're referring to.  That makes future transitions less
problematic.

[0] https://github.com/multiformats/multihash
-- 
brian m. carlson / brian with sandals: Houston, Texas, US
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