+ * In MSG2/MSG3: a list of the peer's public keys that the sender will
+ accept: (i) a 1-byte integer count (ii) that many 5-byte key ids.
+ If not present, implicitly only the special key id 0000000000.
+ * In MSG3/MSG4: an 8-bit integer being an index into the
+ receiver's public key acceptance list, with which the message
+ is signed. If not present, implicitly the key id 00000000000.