>That would violate the non-repudiation properties of the Signal protocol,... Signal achieves non-repudiation by allowing forgery of messages after they have b

>That would violate the non-repudiation properties of the Signal protocol,... Signal achieves non-repudiation by allowing forgery of messages after they have been transferred and verified by the receiver. It is not possible to undo that. You would have to make a special exception at the time the message was transferred which wouldn't be practical in this case. This all assumes that you believe that establishing forgeability actually achieves non-repudiation in the first place. Facebook might want to pass along the signature information anyway.