otr

Off-the-Record Messaging (OTR) is a cryptographic protocol that provides secure, private communication for two-party instant messaging. It is designed to offer confidentiality, message integrity, authentication, and deniable conversations when used over existing chat services such as XMPP, AIM, or IRC clients.

OTR establishes a fresh session key through ephemeral Diffie-Hellman exchanges, delivering forward secrecy so that compromised keys do not reveal past messages. Messages are encrypted and authenticated, protecting content from eavesdroppers and preventing tampering. A distinctive feature is deniable authentication: after a chat ends, transcripts cannot be cryptographically proven to come from a specific participant. Authentication relies on verifying the other party's public key fingerprint, usually through an out-of-band method.

OTR is intended for one-on-one chats and operates as a layer atop standard messaging protocols; it does not hide network metadata such as who is communicating or when. It requires compatible client support and, in practice, is most reliable when both users verify fingerprints during a live handshake. The protocol was introduced in 2004 by Ian Goldberg, Nikita Borisov, and Eric Brewer and has evolved through several versions. Implementations existed in popular clients such as Pidgin with the OTR plugin, Adium, and Gajim, though newer secure-messaging ecosystems have provided alternatives for broader use, including multi-party scenarios.

In other contexts, OTR can stand for Over-the-Road, used in trucking to denote long-haul freight transport.