patatt: main branch gained support for openssh signatures

patatt: main branch gained support for openssh signatures

[Konstantin Ryabitsev]

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Hi, all:

Openssh signing is about to become possible with the latest git release, so
patatt master has gained ability to support openssh signing as well. It
requires OpenSSH 8.0+, so unless you're running one of those LTS distros, you
should already have it available.

The signatures don't really look that dramatically different, though they are
slightly longer than I would have expected for ed25519 keys:

    X-Developer-Signature: v=1; a=openssh-sha256; t=1636987789; l=403;
     i=konstantin@linuxfoundation.org; s=20211115; h=from:subject;
     bh=aWNA6NFmS5xpRH5Gpy45nWiKCOnDOKHOYOV7Y6lyLcU=;
     b=U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgqCBwATTti8v9QsWJB4x1yVA72ozVqlXw
     jcZ/ImRjPZsAAAAGcGF0YXR0AAAAAAAAAAZzaGE1MTIAAABTAAAAC3NzaC1lZDI1NTE5AAAAQGyoMN
     fuL86rhp2CLqjzAoVC9l1sFREfyvnkT/6QpnYht/gQCkAp+KyvWLOaywWPekG5OGMbmwnMu4WOSKmI
     0Qo=
    X-Developer-Key: i=konstantin@linuxfoundation.org; a=openssh;
     fpr=SHA256:movubj27MLZcp0EAsOhlbu3/RJkj1VF9FfHGUsiB4Gw

To add them to the keyring, just add the person's standard openssh
pubkey-formatted file into $KEYRINGPATH/openssh/domain/local/selector.

There are interesting benefits of using openssh as opposed to straight-out
ed25519 keys as generated by patatt:

- openssh keys can be protected by a passphrase and will benefit from
  ssh-agent passphrase caching
- this should, in theory, be fully compatible with -sk ssh keys stored on u2f
  compliant tokens -- though I've yet to test this
- people are more likely to back up openssh keys than their
  ~/.local/share/patatt directory

If you're already using PGP keys, then obviously don't switch. However, this
could be a reasonable option for new people choosing to use patatt.

The support for openssh-signing is in the main branch and I will release
patatt-0.5 after some more testing.

Best regards,
-K

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