I wonder why isn't there a "text messaging mobile app" out there which just uses openPGP signed and encrypted email as the backend. Email is not that far from text messaging anyway.
If an attacker were to record all of a target’s ciphertext traffic over some extended period of time, and then compromise that one key at any point in the future (perhaps by seizing the device it’s on), they would have the ability to decrypt all of the previously recorded ciphertext traffic belonging to the target.