This is populated during signature verification with additional fields
that are present in and covered by the signature. At the moment, it is
only used to record security key-specific options, especially the flags
field.
with and ok markus@
OpenBSD-Commit-ID: 338a1f0e04904008836130bedb9ece4faafd4e49
better match ec25519-sk keys. Discussed with markus@ and Sebastian Kinne
NB. if you are depending on security keys (already?) then make sure you
update both your clients and servers.
OpenBSD-Commit-ID: 53d88d8211f0dd02a7954d3af72017b1a79c0679