Fix server-side authentication with FIDO/U2F sk-* keys#809
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The public keys the server receives on the wire do not have any flags that may be defined in authorized_keys. Therefore the AuthorizedKeyEntriesPublickeyAuthenticator must not include the flags in the keys used for the comparison.
Server-side, authorized_keys may define flags that are relevant for signature verification of sk-* FIDO/U2F keys: no-touch-required and verify-require. UserAuthPublicKey must therefore use the key from the authenticator, which includes these flags, and not the key received on the wire, which doesn't have them. Also be more strict about trailing garbage after the signature.
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Respect server-side attributes that may be set in an
authorized_keysfile:no-touch-requiredandverify-required. These flags exist only server-side; the public keys received in an SSH_MSG_USERAUTH packet do not have them.authorized_keysfile.AuthorizedKeyEntry) so that we can properly check the flags in the signature.Also throw an exception if there is bogus trailing data after the signature in the packet.