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
>> <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.
>
>
> 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?