From: "H. Peter Anvin" <hpa@zytor•com>
To: Stefan Beller <sbeller@google•com>
Cc: "git@vger•kernel.org" <git@vger•kernel.org>
Subject: Re: Migrating away from SHA-1?
Date: Tue, 12 Apr 2016 16:06:53 -0700 [thread overview]
Message-ID: <570D7F8D.9050406@zytor.com> (raw)
In-Reply-To: <CAGZ79kaUN0G7i0GNZgWU7ZzJvWY=k=Rc6tqWvJsTu8gcRhP5bA@mail.gmail.com>
On 04/12/16 16:00, Stefan Beller wrote:
> On Tue, Apr 12, 2016 at 3:38 PM, H. Peter Anvin <hpa@zytor•com> wrote:
>> OK, I'm going to open this can of worms...
>>
>> At what point do we migrate from SHA-1? At this point the cryptoanalysis of
>> SHA-1 is most likely a matter of time.
>
> And I thought the cryptographic properties of SHA1 did not matter for
> Gits use case.
> We could employ broken md5 or such as well.
> ( see http://stackoverflow.com/questions/28792784/why-does-git-use-a-cryptographic-hash-function
> )
> That is because security goes on top via gpg signing of tags/commits.
>
> I am not sure if anyone came up with
> a counter argument to Linus reasoning there?
>
Not true, because what we are signing is a chain of SHA-1s; the
signature is meaningless unless the integrity of the hash chain is
inviolate.
>>
>> For existing repositories we will need to have a migration mechanism. Since
>> we can't modify objects without completely invalidating the cryptographic
>> properties, what I would suggest is that we leave the existing objects as
>> is, with a persistent lookup table from SHA-1 to <new hash>, and have that
>> lookup table signed (e.g. GPG) by the person responsible for converting the
>> repository. This freezes the cryptographic status of the existing SHA-1
>> objects at the time the conversion happens. This is a very good reason to
>> do this before SHA-1 is actually broken In contrast. SHA-2 has been
>> surprisingly resistant to cryptoanalysis, to the point that SHA-3 was
>> motivated by performance and the desire to have a well-tested function based
>> on entirely different principles should a generic attack against the common
>> structure of MD5/SHA-1/SHA-2 would ever be found.
>
> When the kernel moved from BitKeeper to Git, all history was thrown away,
> and started from scratch. The old history could be grafted into the
> repo, if you cared
> though.
>
> I'd propose to go that route again and use a sha1 graft history which
> you can get optionally
> put into your new history for convenience.
>
That was done more for legal reasons than anything else, as far as I
understand. The userbase of git today is also much, much larger than
the userbase for BK ever was.
-hpa
next prev parent reply other threads:[~2016-04-12 23:07 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-12 22:38 Migrating away from SHA-1? H. Peter Anvin
2016-04-12 23:00 ` Stefan Beller
2016-04-12 23:06 ` H. Peter Anvin [this message]
2016-04-12 23:15 ` Jeff King
2016-04-12 23:15 ` David Turner
2016-04-12 23:44 ` Jeff King
2016-04-14 1:53 ` Theodore Ts'o
2016-04-14 16:47 ` Joey Hess
2016-04-14 17:23 ` David Turner
2016-04-14 17:28 ` H. Peter Anvin
2016-04-14 22:40 ` Theodore Ts'o
2016-04-15 2:13 ` Jeff King
2016-04-15 2:18 ` Junio C Hamano
2016-04-15 2:22 ` Jeff King
2016-04-12 23:42 ` Jeff King
2016-04-13 1:03 ` Junio C Hamano
2016-04-13 1:36 ` Jeff King
2016-04-13 1:38 ` H. Peter Anvin
2016-04-13 1:51 ` Duy Nguyen
2016-04-13 1:58 ` H. Peter Anvin
2016-04-15 1:50 ` brian m. carlson
-- strict thread matches above, loose matches on Subject: below --
2016-06-18 2:10 Leo Gaspard
2016-06-18 3:30 ` Eric Wong
2016-06-24 18:17 ` brian m. carlson
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