On Fri, Feb 24, 2017 at 09:32:13AM -0800, Junio C Hamano wrote:
> Ian Jackson <ijackson@chiark.greenend.org.uk> writes:
> 
> > 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 ;-)
> 
> > 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
+1 832 623 2791 | https://www.crustytoothpaste.net/~bmc | My opinion only
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