bug#22883: Authenticating a Git checkout

[Ludovic Courtès]

Hi there!

Ludovic Courtès <address@hidden> skribis:

>> You mentioned that checking signatures on commits is also kinda slow
>> because it’s sequential and not cached.  I don’t know what I really
>> want, but is there perhaps a way to aggregate signatures on past commits
>> so that the client’s work is reduced…?
>
> The caching implemented in 787766ed1e7f0806a98e696830542da528f957bb
> makes things acceptable: the first “make authenticate” run takes a bit
> more than two minutes to check all the commits starting from ‘v1.0.1’,
> but subsequent runs take a few seconds.
>
> I have plans to make things faster (independently of the cache) by doing
> OpenPGP signature verification entirely in Scheme instead of spawning
> ‘gpgv’ every time.  Again, we’ll have to get a prototype before we can
> tell whether it actually is faster.

I’ve been able to resume work on that in the ‘wip-openpgp’ branch:

  5a86b96f54 git-authenticate: Use (guix openpgp).
  4e66563449 openpgp: Add 'string->openpgp-packet'.
  dc0b5d5e01 openpgp: 'lookup-key-by-{id,fingerprint}' return the key first.
  740d804621 openpgp: 'verify-openpgp-signature' looks up by fingerprint when 
possible.
  0157c5ef7f openpgp: Add 'lookup-key-by-fingerprint'.
  31fc7cf080 openpgp: Store the issuer key id and fingerprint in 
<openpgp-signature>.
  c22bede3ce openpgp: Decode the issuer-fingerprint signature subpacket.
  74d0d85e49 DRAFT Add (guix openpgp).

At this stage, ‘make authenticate’ uses the pure-Scheme implementation
(based on Göran Weinholt’s code, heavily modified).  It can authenticate
14K+ commits in ~20s instead of 4m20s on my laptop, which is really nice.

Signature verification in (guix openpgp) does just that: signature
verification.  It does not validate signature and key metadata, in
particular expiration date.  I guess it should at least error out when a
signature creation time is newer than its key expiration time.

It should also reject SHA1 signatures, at least optionally (I haven’t
checked whether our Git history has any of these).

I would very much welcome feedback and advice from an OpenPGP-savvy
person (I’ve Cc’d one to maximize the chances of success :-)).

Next steps:

  • Clean up the (guix openpgp) API a bit, for instance by using proper
    SRFI-35 error conditions.  Perhaps handle v5 packets too.

  • Load the keyring from files in the repo, possibly in a dedicated
    branch.

  • Load the list of authorized keys from the parent of the commit being
    authenticated.

  • Generalize that to channels.

Ludo’.