Consider the following OpenPGP certificate where the primary key is marked as
authentication-capable:
pub rsa2048 2016-11-21 [CA]
E512530B7EFE8A80394B6A7A745D2BF10DE373B5
uid [ full ] ssh://viewsic.mayfirst.org
0 dkg@alice:~$ gpg --export-ssh-key =ssh://viewsic.mayfirst.org
gpg: key "=ssh://viewsic.mayfirst.org" not found: Unusable public key
gpg: export as ssh key failed: Unusable public key
2 dkg@alice:~$
What's unusable about this public key? it's 2048-bit RSA, and it's marked
authentication-capable.
I can get around this by specifying the full fingerprint with a trailing ! but
that shouldn't be necessary since the primary key appears to already have the
authentication key usage flag set.