Multi-Party Off-The Messaging

Abstract

Most cryptographic algorithms provide a means for secret and authentic communication. However, under many circumstances, the ability to repudiate messages or deny a conversation is no less important than secrecy and authenticity. For whistleblowers, informants, political dissidents and journalists - to name a few - it is most important to have means for deniable conversation, where electronic communication must mimic face-to-face private meetings. Off-the-Record Messaging, proposed in 2004 by Borisov, Goldberg and Brewer, and its subsequent improvements, simulate private two-party meetings. Despite some attempts, the multi-party scenario remains unresolved. In this paper, we first identify the properties of multi-party private meetings. We illustrate the differences not only between the physical and electronic medium but also between two- and multi-party scenarios, which have important implications for the design of private chatrooms. We then propose a solution to multi-party off-the-record instant messaging that satisfies the above properties. Our solution is also composable with extensions that provide other properties, such as anonymity. Copyright 2009 ACM.