On 03/06, brian m. carlson wrote:

On Sat, Mar 04, 2017 at 06:35:38PM -0800, Linus Torvalds wrote:

quoted

On Fri, Mar 3, 2017 at 5:12 PM, Jonathan Nieder [off-list ref] wrote:

quoted

This document is still in flux but I thought it best to send it out
early to start getting feedback.

This actually looks very reasonable if you can implement it cleanly
enough. In many ways the "convert entirely to a new 256-bit hash" is
the cleanest model, and interoperability was at least my personal
concern. Maybe your model solves it (devil in the details), in which
case I really like it.

If you think you can do it, I'm all for it.

quoted

Btw, I do think the particular choice of hash should still be on the
table. sha-256 may be the obvious first choice, but there are
definitely a few reasons to consider alternatives, especially if it's
a complete switch-over like this.

One is large-file behavior - a parallel (or tree) mode could improve
on that noticeably. BLAKE2 does have special support for that, for
example. And SHA-256 does have known attacks compared to SHA-3-256 or
BLAKE2 - whether that is due to age or due to more effort, I can't
really judge. But if we're switching away from SHA1 due to known
attacks, it does feel like we should be careful.

I agree with Linus on this.  SHA-256 is the slowest option, and it's the
one with the most advanced cryptanalysis.  SHA-3-256 is faster on 64-bit
machines (which, as we've seen on the list, is the overwhelming majority
of machines using Git), and even BLAKE2b-256 is stronger.

Doing this all over again in another couple years should also be a
non-goal.

I agree that when we decide to move to a new algorithm that we should
select one which we plan on using for as long as possible (much longer
than a couple years).  While writing the document we simply used
"sha256" because it was more tangible and easier to reference.

-- 
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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-- 
Brandon Williams