Enumeration SignaturePolicy

On the producing side:

• Build messages without the signature, key, from and seqno fields.
• The corresponding protobuf key-value pairs are absent from the marshalled message, not just empty.

On the consuming side:

• Enforce the fields to be absent, reject otherwise.
• Propagate only if the fields are absent, reject otherwise.
• A message_id function will not be able to use the above fields, and should instead rely on the data field. A commonplace strategy is to calculate a hash.

On the producing side:

• Build messages with the signature, key (from may be enough for certain inlineable public key types), from and seqno fields.

On the consuming side:

• Enforce the fields to be present, reject otherwise.
• Propagate only if the fields are valid and signature can be verified, reject otherwise.