Ofer Hermoni

Deniability — an alibi for users in P2P networks

Peer to peer file sharing is dasiaboomingpsila, but meanwhile censorship of these networks and prosecution of users that share censored content are growing just as quickly. In this paper, we propose a novel notion of deniability as an easy and efficient method for users to avoid censorship and prosecution. The fundamental concept is that a given data element, which contains controversial or censored content, is also associated with neutral-content material. Hence, even a powerful adversary capable of monitoring all communication in the network and viewing the internal state of participating hosts is unable to prove a link between censored content and a user. The communication overhead required to retrieve a document is only four times greater than what is needed in a standard network. The storage required for a document is only twice as large as the document itself. Deniability is an elegant alternative to user anonymity in P2P file sharing networks. Systems that provide anonymity for users typically require greater overhead and do not guarantee anonymity against powerful, real-world adversaries.

Rendezvous tunnel for anonymous publishing: clean slate and tor based designs

Anonymous communication, and in particular anonymous Peer-to-Peer (P2P) file sharing systems, have received considerable attention in recent years. In a P2P file sharing system, there are three types of participants: publishers that insert content into the system, servers that store content, and readers that retrieve content from the servers. Existing anonymous P2P file sharing systems confer partial anonymity. They provide anonymity to participant pairs, such as servers and readers or publishers and readers, but they do not consider the anonymity of all three types of participants. In this work we propose two solutions for anonymous P2P file sharing systems. Both of our solutions provide anonymity to all three types of participants. The proposed solutions are based on indexing by global hash functions (rather than an index server), dispersal of information, and three anonymity tunnels. Each anonymity tunnel is designed to protect the anonymity of a different user (publisher, server, or reader). In both solutions the reader and publisher tunnels are sender anonymity tunnels. In the first solution the third tunnel is a rendezvous tunnel, constructed by means of a random walk and terminating at the server. In the second solution, which is based on Tor, the third tunnel is built using Tors hidden services. The first solution preserves anonymity in the presence of a semi-honest adversary that controls a limited number of nodes in the system. The second solution is based on Tor primitives, coping with the same adversary as that assumed in Tor. The second solution enhances Tor, ensuring publisher, server, and reader anonymity.

Rendezvous tunnel for anonymous publishing

Many anonymous peer-to-peer (P2P) file sharing systems have been proposed in recent years. One problem that remains open is how to protect the anonymity of all participating users, namely, reader, server and publisher. In this work we propose a novel solution for a P2P file sharing system. Our solution provides overall anonymity to all participating users. Servers in our system store shares of documents, and each share is reached through a rendezvous tunnel between the server and an address given by a hash of the documents name. To publish a document, the publisher first divides the document into shares, for each share finds the address of the entrance to the tunnel by hashing the documents name. Next, the publisher uses anonymous communication to reach the entrance of the rendezvous tunnel. We then use a random walk and an anonymous key exchange scheme to set keys along the rendezvous tunnel. The publisher finishes by inserting the shares into the servers through the rendezvous tunnels. A reader wanting to retrieve the document operates in a similar manner. The reader finds the address of the entrance to the rendezvous tunnels by hashing the documents name. Then, the reader uses anonymous communication to reach the entrance of the tunnels, retrieves the shares anonymously and reconstructs the document. The novelty of this work is threefold. First, we introduce an anonymous key exchange protocol secure against an honest but curious adversary. The anonymity of the protocol is proved on the basis of the Decisional Diffie Hellman (DDH) problem. Second, we propose two solutions to build the rendezvous tunnel: basic and advanced. The basic solution is straightforward, while the advanced solution is based on the key exchange protocol. In the advanced solution, the key exchange is done between the publisher and each user along the rendezvous tunnel. Third, the rendezvous tunnel is used as a building block for an anonymous P2P file sharing system that provides anonymity to all participating users.

Poster: arbitrators in the security infrastructure, supporting positive anonymity

Traditional public key infrastructure is an example for basing the security of communication among users and servers on trusting a Certificate Authority (CA) which is a Trusted Authority (TA). A traditional, centralized CA or TA should only be involved in a setup stage for communication, or risk causing a bottleneck. Peer to peer assistance may replace the CA during the actual communication transactions. We introduce such assistants that we call arbitrators. Arbitrators are semi-trusted entities that facilitate communication or business transactions. The communicating parties, users and servers, agree before a communication transaction on a set of arbitrators that they trust (reputation systems may support their choice). Then, the arbitrators receive resources, e.g. a deposit, and a service level agreement between participants such that the resources of a participant are returned if and only if the participant acts according to the agreement. We demonstrate the usage of arbitrators in the scope of conditional (positive) anonymity. A user may interact anonymously with a server as long as the terms for anonymous communication are honored. In case the server finds a violation of the terms, the server proves to the arbitrators that a violation took place and the arbitrators publish the identity of the user. Since the arbitrators may be corrupted, the scheme ensures that only a large enough set of arbitrators may reveal users identity, which is the deposited resource in the case of conditional anonymity.

Anonymity Scheme for Interactive P2P Services

The flourishing of Peer-to-Peer (P2P) applications has encouraged extensive research on end-to-end anonymity. In particular, interactive services such as VoIP and instant messaging are becoming extremely popular and require low latency. Preserving the anonymity of users in such services is becoming a major challenge. While existing schemes provide anonymity for the communicating parties; they incur high latency and are not suitable for interactive services. Even designs that attempt to reduce the latency, employ an encryption scheme that involves a comparatively high computation overhead and delay. In this paper, we describe a novel low-latency P2P scheme that provides anonymity for both senders and receivers while adhering to low bounds of delay, communication and bandwidth overheads. The design offers resistance against various attacks including a coalition of malicious peers. It has inherited scalability and fault tolerance and is applicable for various interactive P2P services.

Digital arbitration for trusted communication

We introduce the notion of digital-arbitration which enables resolving disputes between servers and users with the aid of arbitrators. Arbitrators are semi-trusted entities in the social network that facilitate communication or business transactions. The communicating parties, users and servers, agree before a communication transaction on a set of arbitrators they trust (reputation systems may support their choice). Then, the arbitrators receive a resource, e.g., a deposit, and a terms-of-use agreement between participants such that the resource of a participant is returned if and only if the participant acts according to the agreement.We demonstrate the usage of arbitrators in the scope of conditional anonymity. A user interacts anonymously with a server as long as the terms for anonymous communication are honored. If a server identifies a violation of the terms, it proves to the arbitrators that a violation took place and the arbitrators publish the identity of the user.

Brief announcement : arbitrators in the security infrastructure

We introduce the notion of digital arbitration which enables resolving disputes between servers and users with the aid of arbitrators. Arbitrators are semi-trusted entities in a social network that facilitate communication or business transactions. The communicating parties, users and servers, agree before a communication transaction on a set of arbitrators that they trust (reputation systems may support their choice). Then, the arbitrators receive digital goods, e.g. a deposit, and a terms of use agreement between participants such that the goods of a participant are returned if and only if the participant acts according to the agreement.