&gt; Strong, deniable, end-to-end encryption should be mandatory.<p>Strong end-to-end encryption with perfect forward secrecy should be mandatory.  Deniable authentication (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Deniable_authentication" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Deniable_authentication</a>), however, seems like a potentially interesting option but not one that the protocol should mandate.  Sometimes you <i>do</i> want authentication that remains valid after the conversation ends, so you can subsequently authenticate the messages in it.