PubSub action request and response messages need non-repudiation signatures.
This would be useful for other messages as well.
Should work for JSON and UADP

Text removed from 1.05.04 Draft:
The WriterGroup also specifies security requirements for the RequestMessage. If the WriterGroup SecurityMode indicates that signing is required then a non-repudiation Signature shall be attached to the Request message. WriterGroup SecurityGroupId identifies the Authority Message containing the TrustList used by the Responder to determine if the Requestor is authorized to invoke the Action.