Improved group off-the-record messaging

Off-the-Record Messaging (OTR) is an online analogy of face-to-face private chat -- messages are confidential and authenticated at the time of the conversation, but cannot later be used to prove authorship. The original OTR protocol is limited to two parties, and is extended by multi-party OTR (mpOTR) to the group chat setting. In doing this, mpOTR unintentionally weakens the security properties provided by its two-party predecessor. We propose an improved group OTR (GOTR)protocol that provides unconditional repudiability, and show how to obtain data origin authentication given this level of repudiability. GOTR resists network failure, colluding and independent malicious insiders, and provides efficient and flexible membership management. We analyze the security properties and performance of GOTR, and present measurement results of a proof-of-concept implementation of GOTR.