Conversation signature IDs

[redlizard]

Currently, chat messages contain a signature over a payload that includes the number of messages sent using that key. This is a simple way to avoid any replay attacks, as replaying an earlier chat message by a user is considered invalid (it has an incorrect message number).

This scheme only applies to chat messages, not conversation messages. As a consequence, conversation messages are vulnerable to replay attacks. The most critical parts of the protocol have data covered by the signature to prevent this, but not all messages do. It's an easy fix to avoid the entire class of problems by adding a similar message-ID field to conversation messages.