“Off-the-Record Messaging, commonly referred to as OTR, is a cryptographic protocol that provides strong encryption for instant messaging conversations. OTR uses a combination of the AES symmetric-key algorithm, the Diffie–Hellman key exchange, and the SHA-1 hash function. In addition to authentication and encryption, OTR provides perfect forward secrecy and malleable encryption.
The primary motivation behind the protocol was providing deniability for the conversation participants while keeping conversations confidential, like a private conversation in real life, or off the record in journalism sourcing.”
The Guardian Project hosts a fantastic how-to guide to chatting securely on Android with ChatSecure.
Threema is not recommend by PRISM Break as it is closed source software. Freely available source code is a necessary condition for privacy and security.