bug#22883: Authenticating a Git checkout

[Christopher Allan Webber]

Ludovic Courtès writes:

>>> Second, even if it did, it would be a shallow check: as Mike notes in
>>> <https://mikegerwitz.com/papers/git-horror-story> with the ‘signchk’
>>> script, you actually have to traverse the whole commit history and
>>> authenticate them one by one.  But that’s OK, it runs in presumably less
>>> than a minute on a repo the size of Guix’s, and we could also stop at
>>> signed tags to avoid redundant checks.
>>
>> Practically speaking, that's probably fine, though note that a signed
>> tag is just a signed hash of the commit it points to (with some
>> metadata), so you're trusting the integrity of SHA-1 and nothing
>> more.
>>
>> With that said, the tag points to what will hopefully be a signed
>> commit, so if you verify the signature of the tag _and_ that commit,
>> that'd be even better.  Git's use of SHA-1 makes cryptographic
>> assurances difficult/awkward.
>>
>> An occasional traversal of the entire DAG by, say, a CI script would
>> provide some pretty good confidence.  I wouldn't say it's necessary for
>> every pull.
>
> Agreed.

One theoretical optimization: if I verify the DAG, could I store
somewhere that I've verified from commit cabba6e and upward already, so
the next time I verify it only has to verify the new commits?

Mostly makes sense if we're already going down the only mildly
crazypants direction of implementing our own tooling :)

 - Chris