bug#43421: Encoding issue in exported archive signatures

[Ludovic Courtès]

Following the ‘guix authenticate’ in commit
64cf660f872fb7aaf0d2b463e45b4c756297f743¹, I’m observing encoding
issues:

  guix archive --export \
    /gnu/store/3p5wcw2a0844rbcmlrqfjx8bx7b7gq34-r-rvest-0.3.6-guile-builder

yield an archive with this signature:

--8<---------------cut here---------------start------------->8---
(signature
 (data
  (flags rfc6979)
  (hash sha256 
#1DEE0418AF5FD8A05D2142290BA03735176FA27BB68B3A02977C774EA3DBDAEC#)
  )
 (sig-val
  (ecdsa
   (r #072B8E5C6B84D4ED469EC2CF63103621602E9AF3902E454CAD49CFA6BDE2FBF0#)
   (s "~%*Øw2%YZ»+yvc*¤Ì44C;RM\t3EQIp<ü")
   )
  )
 (public-key
  (ecc
   (curve Ed25519)
   (q #8D156F295D24B0D9A86FA5741A840FF2D24F60F7B6C4134814AD55625971B394#)
   )
  )
 )
--8<---------------cut here---------------end--------------->8---

Notice the ‘s’ field of the signature.

The problem does not occur systematically: it depends on the byte string
(libgcrypt encodes Latin-1ish strings as strings and other strings as
hex sequences.)  The problem is similar to <https://issues.guix.gnu.org/17312>.

The interesting bit is that this archive can be correctly ingested by a
new daemon, but it fails signature verification with an older daemon.

Ludo’.

¹ https://issues.guix.gnu.org/43340