KEYS: Allow authentication data to be stored in an asymmetric key

Allow authentication data to be stored in an asymmetric key in the 4th
element of the key payload and provide a way for it to be destroyed.

For the public key subtype, this will be a public_key_signature struct.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/include/keys/asymmetric-type.h b/include/keys/asymmetric-type.h
index 59c1df9..70a8775 100644
--- a/include/keys/asymmetric-type.h
+++ b/include/keys/asymmetric-type.h
@@ -23,9 +23,10 @@
  * follows:
  */
 enum asymmetric_payload_bits {
-	asym_crypto,
-	asym_subtype,
-	asym_key_ids,
+	asym_crypto,		/* The data representing the key */
+	asym_subtype,		/* Pointer to an asymmetric_key_subtype struct */
+	asym_key_ids,		/* Pointer to an asymmetric_key_ids struct */
+	asym_auth		/* The key's authorisation (signature, parent key ID) */
 };
 
 /*