Multi-party Off-the-Record Messaging

Ian Goldberg, Berkant Ustaoglu, Matthew Van Gundy, and Hao Chen

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 elect
ronic medium but also between two- and multi-party scenarios, which have importa
nt 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.