Re: Git and SHA-1 security (again)
From: Duy Nguyen <hidden>
Date: 2016-07-19 18:05:39
On Tue, Jul 19, 2016 at 7:59 PM, David Lang [off-list ref] wrote:
On Tue, 19 Jul 2016, Duy Nguyen wrote:quoted
On Tue, Jul 19, 2016 at 7:34 PM, David Lang [off-list ref] wrote:quoted
On Tue, 19 Jul 2016, Duy Nguyen wrote:quoted
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?The set of all heads at the time the conversion happens (maybe plus all the real tags). We can make an octopus merge commit to cover all the heads, then it can be the reference point.so to make sure I'm understanding this, anything not reachable from that merge must be the new hash, correct? Including forks, merges, etc that happen from earlier points in the history.
Yes everything except that merge and everything reachable from it, the whole old clone, basically. -- Duy