Re: How safe are signed git tags? Only as safe as SHA-1 or somehow safer?

From: Jeff King <peff@peff.net>
Date: Nov 24, 2014, 15:51 UTC
Message-ID: <20141124155108.GC25912@peff.net>
In-Reply-To: <54730546.7000200@drmicha.warpmail.net>

On Mon, Nov 24, 2014 at 11:15:34AM +0100, Michael J Gruber wrote:

Show 8 quoted lines

> > I wonder if we can have an option to sign all blob content of the tree
> > associated to a commit, and the content of parent commit(s). It's more
> > expensive than signing just commit/tag content. But it's also safer
> > without completely ditching SHA-1.
> > 
> 
> This amounts to hashing the blob content with whatever hash you told
> your gpg to use (hopefully not sha1 ;) ) and signing that.

Right. You could also create a graph of SHA-256 (or whatever) object hashes and sign that. I.e., create a parallel to git's trees using SHA-256 and include a single:

  object-256 ....

line in the tag header. That still involves re-hashing all of the data, but it would at least be possible to cache (i.e., a mapping of SHA-1 to SHA-256 hashes). Of course one way to keep that caching layer up to date would be to just calculate the SHA-256 along with the SHA-1 whenever we create an object. And then you can sprinkle SHA-256 links in other places, too, like commit objects.

And now you are halfway down the road to a combined SHA-1/SHA-256 git. :)

The tricky thing is fitting the extra hash into the tree objects. And of course the rules for actually generating and/or sending extra objects.

-Peff