, 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
, OTR provides perfect forward secrecy
and deniable 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. This is in contrast with the majority of cryptography tools which resemble more a signed writing on paper, which can be later used as a record to demonstrate the communication event, the participants, and the topic of communication. Unfortunately, in most cases people using ordinary cryptography software are not aware of this and in most cases they would be better served by OTR tools instead. Hence the initial introductory paper was named "Off-the-Record Communication, or, Why Not To Use PGP".
The OTR protocol was designed by cryptographers Ian Goldberg and Nikita Borisov. They provide a client library to facilitate support for instant messaging client developers who want to implement the protocol and a special OTR-proxy for AIM, ICQ, and .Mac clients which support proxies.
In addition to providing encryption and authentication — features also provided by typical public-key cryptography suites, such as PGP
, and X.509
) — OTR also offers some less common features:
- Perfect forward secrecy: Messages are only encrypted with temporary per-message AES keys, negotiated using the Diffie-Hellman key exchange protocol. The compromise of any long-lived cryptographic keys does not compromise any previous conversations, even if an attacker is in possession of ciphertexts.
- Deniable authentication: Messages in a conversation do not have digital signatures, and after a conversation is complete, anyone is able to forge a message to appear to have come from one of the participants in the conversation, assuring that it is impossible to prove that a specific message came from a specific person.
As of OTR 3.1 the protocol supports mutual authentication of users using a shared secret through the socialist millionaire protocol. This feature makes it possible for users to verify the identity of the remote party and avoid a man in the middle attack without the inconvenience of manually comparing public key fingerprints through an outside channel.
Due to limitations of the protocol, OTR does not currently support multi-user group chat or encrypted file transfers but may be implemented in future. Support for encrypted audio or video is not planned.
These clients support Off-the-Record Messaging out of the box.
The following clients require a plug-in to use Off-the-Record Messaging. Plugin support allows use of OTR with all of a client's implemented instant messaging protocols (e.g. OSCAR
For those clients which have no native OTR support, a GUI proxy is available. That means that the messages are sent to the proxy unencrypted and get encrypted while they "flow" through this locally installed and running application called a proxy. Presently the proxy provided by the OTR-project supports only the OSCAR-protocol, thus it can be used for .Mac, ICQ, and AIM. The OTR proxy is capable of SOCKS5
], and [
Some .Mac, ICQ, and AIM clients that support proxies, but don't support OTR natively: