Off-the-Record Messaging

The primary motivation behind the protocol was providing deniable authentication 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 cryptography tools that produce output which can be later used as a verifiable record of the communication event and the identities of the participants. 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 and released on 26 October 2004. They provide a client library to facilitate support for instant messaging client developers who want to implement the protocol. A Pidgin and Kopete plugin exists that allows OTR to be used over any IM protocol supported by Pidgin or Kopete, offering an auto-detection feature that starts the OTR session with the buddies that have it enabled, without interfering with regular, unencrypted conversations. Version 4 of the protocol is currently been designed by a team led by Sofía Celi and Ola Bini, and reviewed by Nik Unger and Ian Goldberg. This version aims to provide online and offline deniability, to update the cryptographic primitives, and to support out-of-order delivery and asynchronous communication. OTR was presented in 2004 by Nikita Borisov, Ian Avrum Goldberg, and Eric A. Brewer as an improvement over the OpenPGP and the S/MIME system at the 'Workshop on Privacy in the Electronic Society' (WPES). The first version 0.8.0 of the reference implementation was published on 21 November 2004. In 2005 an analysis was presented by Mario Di Raimondo, Rosario Gennaro, and Hugo Krawczyk that called attention to several vulnerabilities and proposed appropriate fixes, most notably including a flaw in the key exchange. As a result, version 2 of the OTR protocol was published in 2005 which implements a variation of the proposed modification that additionally hides the public keys. Moreover, the possibility to fragment OTR messages was introduced in order to deal with chat systems that have a limited message size, and a simpler method of verification against man-in-the-middle attacks was implemented. In 2007 Olivier Goffart published mod_otr for ejabberd, making it possible to perform man-in-the-middle attacks on OTR users who don't check key fingerprints. OTR developers countered this attack by introducing socialist millionaire protocol implementation in libotr. Instead of comparing key checksums, knowledge of an arbitrary shared secret can be utilised for which relatively low entropy can be tolerated by using the socialist millionaire protocol. Version 3 of the protocol was published in 2012. As a measure against the repeated reestablishment of a session in case of several competing chat clients being signed on to the same user address at the same time, more precise identification labels for sending and receiving client instances were introduced in version 3. Moreover, an additional key is negotiated which can be used for another data channel. Several solutions have been proposed for supporting conversations with multiple participants. A method proposed in 2007 by Jiang Bian, Remzi Seker, and Umit Topaloglu uses the system of one participant as a 'virtual server'. The method called 'Multi-party Off-the-Record Messaging' (mpOTR) which was published in 2009 works without a central management host and was introduced in Cryptocat by Ian Goldberg et al. In 2013, the Signal Protocol was introduced, which is based on OTR Messaging and the Silent Circle Instant Messaging Protocol (SCIMP). It brought about support for asynchronous communication ('offline messages') as its major new feature, as well as better resilience with distorted order of messages and simpler support for conversations with multiple participants. OMEMO, introduced in an Android XMPP client called Conversations in 2015, integrates the Double Ratchet Algorithm used in Signal into the instant messaging protocol XMPP ('Jabber') and also enables encryption of file transfers. In the autumn of 2015 it was submitted to the XMPP Standards Foundation for standardisation. Currently, version 4 of the protocol is been designed. It was presented by Sofía Celi and Ola Bini on PETS2018.