In my personal opinion: if you want to communicate securely, with a very low probability of eavesdropping - use voice chat in any of modern games, on any platform. On both PS4 and X1, voice chat communication has to be encrypted(it's a compliance requirement), but at the same time there is no requirement that you have to implement a server relay - so a lot of games don't bother doing it, and it's a straight up P2P mesh implementation. You have encrypted, P2P traffic, not stored anywhere, that you can create an anonymous account to use.