Re: Git and SHA-1 security (again)

[Duy Nguyen <pclouds@gmail.com>]

On Tue, Jul 19, 2016 at 9:18 AM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
>> But we can recreate SHA-1 from the same content and verify GPG, right?
>> I know it's super expensive, but it feels safer to not carry SHA-1
>> around when it's not secure anymore (I recall something about
>> exploiting the weakest link when you have both sha1 and sha256 in the
>> object content). Rehashing would be done locally and is better
>> controlled.
>
> You could. But how would you determine whether to recreate the commit
> object from a SHA-1-ified version of the commit buffer? Fall back if the
> original did not match the signature?

Any repo would have a cut point when they move to sha256 (or whatever
new hash), if we can record this somewhere (e.g. as a tag or a bunch
of tags, or some dummy commits to mark the heads of the repo) then we
only verify gpg signatures _in_ the repository before this point.

> That would pose at least these two problems:
>
> 1. The point of a signature is trust. If all of a sudden the signature
> does not match what is supposedly signed, that trust is broken.
>
> 2. The point of going to a stronger hash is to increase the trust. If
> any developer could decide to sign the SHA-1-ified version of any future
> commit, and Git validating it, it would be even worse than not switching
> to a new hash: it would leave us open to collision attacks *and* pretend
> that we prevented such attacks.

GPG signatures are still valid on the old repo (we will keep old repos
around forever, I suppose). And because they sign on the "weak" hash,
sha1, at some point they will be broken (but until then we can still
regenerate sha1 and verify locally). When sha1 is broken, GPG
signatures of the past can't be trusted anymore.

If people care enough about the past, they should re-sign (at least
for tags). Commits can be re-signed by the person who does the
conversion. Yes you have to trust that person. Sort of a painful fresh
start, with hopefully better security.
-- 
Duy