OTR

Off-the-Record Messaging provides deniable encryption, perfect forward secrecy, and authentication for instant messaging protocols.

OTR (Off-the-Record Messaging) secures real-time communications by combining AES symmetric-key encryption, the Diffie-Hellman key exchange, and SHA-1 hash functions. Unlike PGP, it ensures deniability: digital signatures are handled so that while participants are certain of each other's identity during the session, no third party can prove a specific individual authored a message after the fact. Version 4 (OTRv4) continues this legacy by integrating the Double Ratchet algorithm and Ed448 curves to maintain high-security standards against modern cryptanalysis.

https://otr.cypherpunks.ca/