Re: Git and SHA-1 security (again)

On Tue, 19 Jul 2016, Duy Nguyen wrote:

On Tue, Jul 19, 2016 at 9:18 AM, Johannes Schindelin
[off-list ref] wrote:

quoted

quoted

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.

remember that a repo doesn't have a single 'now', each branch has it's own head, 
and you can easily go back to prior points and branch off from there.

Since timestamps in repos can't be trusted (different people's clocks may not be 
in sync), how would you define this cutoff point?

David Lang