+ o Merge the KE1 and KE2 payloads into one KE payload that has
+ the `signature' field. Provide it only if the perty is
+ required to do authentication.
+ o Add MUTUAL_AUTH flag to indicate that the party must perform
+ authentication (to sign with their private key). Initiator
+ may set it to indicate that it can authenticate but responder
+ MAY require for the initiator to do authentication by setting
+ the flag at the KE Start Payload reply phase. The responder
+ performs authentication always as now as well.