That being said, I'm not certain encryption is an entirely solved problem for the case of multiple devices, including web clients, or for large public groups. (WhatsApp only supports a single client -- their web interface attaches to the phone -- and their group chats are limited to 256 members.) I'm not sure it can be solved under the current model Telegram uses for authorizing devices, as the server can authorize a device to access an account, and any non-secret chats it was involved in, without the involvement of any previously signed-in devices.