Off-the-Record Messaging protocol

Off-the-Record Messaging (OTR) is a cryptographic protocol that provides encryption for instant messaging conversations. OTR uses a combination of AES symmetric-key algorithm with 128 bits key length, the Diffie–Hellman key exchange with 1536 bits group size, and the SHA-1 hash function.

Source: Wikipedia — Off-the-Record Messaging protocol (CC BY-SA 4.0)