Re: Migrating away from SHA-1?

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On Sat, Jun 18, 2016 at 03:10:27AM +0100, Leo Gaspard wrote:
> First, sorry for not having this message threaded: I'm not subscribed to
> the list and haven't found a way to get a Message-Id from gmane.

Sorry it's taken so long to get back to this.  I've been at a
conference.

> So, my questions to the git team:
>  * Is there a consensus, that git should migrate away from SHA-1 before
> it gets a collision attack, because it would mean chosen-prefix
> collision isn't far away and people wouldn't have the time to upgrade?

I plan on adding support for a new hash as soon as that's possible, but
I don't have a firm timeline.  This is a volunteer effort in my own
limited free time.

>  * Is there a consensus, that Peter Anvin's amended transition plan is
> the way to go?

I'm not planning on changing algorithms in the middle of a repository.
This will only be available on new or imported repositories.

My current thinking on proposed algorithms is SHA3-256 or BLAKE2b-256.
The cryptanalysis on SHA-256 indicates that it may not be a great
long-term choice, and I expect people won't want to change algorithms
frequently.

If time becomes extremely urgent, we can always add support for a
160-bit hash first (e.g. BLAKE2b-160) and then finish the object_id
transition later as it becomes convenient.  I'd like to avoid that,
though.

>  * If the two conditions above are fulfilled, has work started on it
> yet? (I guess as Brian Carlson had started his work 9 weeks ago and he
> was speaking about working on it on the week-end he should have finished
> it now, so excluding this)

It takes a long time to get a patch series through.  I'm rather busy and
don't always have time to rebase and address issues during the week.

>  * If the two first conditions are fulfilled, is there anything I could
> do to help this transition? (including helping Brian if his work hasn't
> actually ended yet)

You're welcome to send patches if you like.  I try to avoid areas I know
are under heavy development, like the refs code.
-- 
brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | https://www.crustytoothpaste.net/~bmc | My opinion only
OpenPGP: https://keybase.io/bk2204

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