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Multi-Party Off-The-Record Messaging

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Multi-Party Off-The-Record Messaging Multi-party Off-the-Record Messaging Ian Goldberg Berkant Ustaoglu˘ David R. Cheriton School of Computer Science NTT Information Sharing Platform Laboratories University of Waterloo Tokyo, Japan Waterloo, ON, Canada [email protected] [email protected] Matthew D. Van Gundy Hao Chen Department of Computer Science Department of Computer Science University of California, Davis, CA, USA University of California, Davis, CA, USA [email protected] [email protected] ABSTRACT Keywords Most cryptographic algorithms provide a means for secret Privacy, deniability, multi-party, instant messaging and authentic communication. However, under many cir- cumstances, the ability to repudiate messages or deny a con- 1. MOTIVATION versation is no less important than secrecy and authentic- ity. For whistleblowers, informants, political dissidents and The Internet presents a novel means of communication journalists | to name a few | it is most important to have | instant messaging (IM), where users can engage in ac- means for deniable conversation, where electronic commu- tive conversations across great distances. However, IM lacks nication must mimic face-to-face private meetings. Off-the- certain fundamental security properties that a physical pri- Record Messaging, proposed in 2004 by Borisov, Goldberg vate conversation can provide. Impersonation, eavesdrop- and Brewer, and its subsequent improvements, simulate pri- ping and information copying are all trivial attacks in IM. vate two-party meetings. Despite some attempts, the multi- Online communication systems commonly provide three party scenario remains unresolved. properties: confidentiality, authentication and non-repudia- In this paper, we first identify the properties of multi- tion. Confidentiality and authentication are expected traits party private meetings. We illustrate the differences not of face-to-face conversations, but non-repudiation clashes only between the physical and electronic medium but also with the expectations for private communication. Non-re- between two- and multi-party scenarios, which have impor- pudiation denotes a receiver's ability to prove to a third tant implications for the design of private chatrooms. We party, possibly a judge, that the sender has authored a mes- then propose a solution to multi-party off-the-record instant sage. Although desirable under many circumstances, non- messaging that satisfies the above properties. Our solution repudiation is the very property journalists, dissidents or is also composable with extensions that provide other prop- informants wish to avoid. erties, such as anonymity. Borisov, Goldberg and Brewer [6] argued that instant mes- saging should mimic casual conversations. Participants of a Categories and Subject Descriptors casual talk can deny their statements in front of outsiders, and can sometimes deny having taken part in the talk at all. K.4.1 [Management of Computing and Information The authors presented a mechanism called Off-the-Record Systems]: Public Policy Issues|Privacy; E.3 [Data]: Data Messaging (OTR) that allows two-party private conversa- Encryption; K.6.5 [Management of Computing and In- tions using typical IM protocols. OTR aims to provide con- formation Systems]: Security and Protection|Authenti- fidentiality, authentication, repudiation and forward secrecy, cation; H.4.3 [Information Systems Applications]: Com- while being relatively simple to employ. munication Applications|Computer conferencing, telecon- Despite its good design, OTR has limitations, the most ferencing, and videoconferencing; C.2.2 [Computer- important of which is that it can serve only two users. Hence Communication Protocols]: Network Protocols|Appli- it is not suitable for online multi-party conversations, com- cations monly enjoyed by casual users via Internet Relay Chat (IRC), General Terms by open-source software developers, and by businesses that cannot afford confidential meetings across vast distances [4, Security, Algorithms x2.3]. It is non-trivial to extend OTR to allow for multi- party conversations, as OTR uses cryptographic primitives designed for two parties. For example, OTR uses message Permission to make digital or hard copies of all or part of this work for authentication codes (MACs) to provide authenticity. While personal or classroom use is granted without fee provided that copies are for two parties MACs can provide a deniable authentica- not made or distributed for profit or commercial advantage and that copies tion mechanism, MACs do not provide origin authentication bear this notice and the full citation on the first page. To copy otherwise, to when used by more than two parties. republish, to post on servers or to redistribute to lists, requires prior specific Bian, Seker and Topaloglu [4] proposed a method for ex- permission and/or a fee. CCS’09, November 9–13, 2009, Chicago, Illinois, USA. tending OTR for group conversation. The crux of their so- Copyright 2009 ACM 978-1-60558-352-5/09/11 ...$5.00. lution is to designate one user as the \virtual server". While this may be feasible under certain circumstances, it devi- 1.2 Outline ates from the original OTR goal, which is to mimic private In x2 we identify the relevant properties of private meet- conversations. In private group conversations there is no ings and how they apply to IM. x3 describes the different virtual server responsible for smooth meetings. Moreover, players of our model for private communication; we focus the server becomes an enticing target for malicious parties. on the different adversaries and their goals. x4 outlines our Finally, the server has to be assumed honest, as a dishonest solution at a high level and shows that we have achieved the server could compromise both the confidentiality and the goals of private conversations. Due to space limitations we integrity of all messages sent during a chat session. only touch upon the many cryptographic primitives and the In this work, we present a multi-party off-the-record pro- formal definitions we use in this paper. We conclude in x5. tocol (mpOTR), which provides confidentiality, authenticity and deniability for conversations among an arbitrary num- 2. PRIVATE CHATROOMS ber of participants. Using our protocol, an ad hoc group of individuals can communicate interactively without the need 2.1 Confidentiality for a central authority. We identify the important traits In meetings a user A^ is willing to reveal information to of multi-party authentication for users, for messages and chatroom members but not outsiders. Hence chatroom mes- for chatrooms that share users; that is, we take into ac- sages need to remain hidden from the wider community. count that two or more users may concurrently share more In private physical communication, should a new party ap- than one chatroom with different peers. When considering proach, the participants can \detect" the newcomer and take privacy properties, we allow malicious insiders and identify appropriate actions. their goals. These multi-party chatroom properties present On the Internet eavesdropping cannot be detected as eas- new challenges that were not addressed in previous work. ily; however, there are ways to guard against casual eaves- An OTR transcript reveals that a user at some point com- droppers. Cryptographic algorithms can assure parties that municated with someone. Our mpOTR protocol carries de- observers looking at the transmitted conversation packets niability further by allowing the user to deny everything ex- are left in dark about the communicated content. That is, cept, by virtue of being part of the system, that they were the transcript gives an eavesdropper no additional knowl- willing at some point to engage in a conversation. In fact, edge above information about lengths of messages and traffic it is unclear how users can deny the latter at all because patterns, beyond what the eavesdropper could have deduced by using the Internet, they already indicate their intent and without having seen the encrypted messages. ability to engage with others. In this sense, mpOTR is closer than OTR to simulating deniability in private conversations 2.2 Entity authentication in the physical world: anyone could take or have taken part In a face-to-face meeting we identify peers via their ap- in a private conversation, but that person can plausibly deny pearances and physical attributes. By contrast, on the Inter- ever having done so. net a user proves to another user knowledge of some secret identifying information, a process known as entity authenti- cation. The basic goal of entity authentication is to provide evi- dence that a peer who presents public key S also holds the 1.1 Related work B^ corresponding private key sB^ . For example, suppose Alice While not the first to address security in instant messag- provides a challenge to Bob. If Bob can compute a response ing, Borisov, Goldberg and Brewer [6] popularized its pri- that can only be computed by an entity possessing sB^ , then vacy aspects, partly due to their now-popular open-source Bob successfully authenticates himself to Alice. This type plugin. Subsequently, more research was devoted to IM; in of authentication is limited in the sense that Bob only shows fact, the original proposal was found to contain errors [10], knowledge of sB^ . If Bob wants to claim any further creden- which were repaired in a subsequent version of OTR. tials like \classmate Bob", then Alice would need additional On a high level there are two approaches to secure IM. proofs. Two-party entity authentication has been studied in Clients can establish their connections via a centralized server the setting of OTR by Alexander and Goldberg [1, x4 and and rely on the server for security and authentication [16]. x5]; their solution is suitable for pairwise authentication. Alternatively, participants can use shared knowledge to au- The entity authentication goal for mpOTR is to provide thenticate each other [1]. OTR, which aims to simulate ca- a consistent view of chatroom participants: each chat par- sual conversations, is closer to the second solution. ticipant should have the same view of the chatroom mem- While there is a wide literature on IM (see [15, x2.1] for an bership.
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