cMix: Mixing with Minimal Real-Time Asymmetric Cryptographic Operations

We introduce cMix, a new approach to anonymous communications. Through a precomputation, the core cMix protocol eliminates all expensive realtime public-key operations --- at the senders, recipients and mixnodes --- thereby decreasing real-time cryptographic latency and lowering computational costs for clients. The core real-time phase performs only a few fast modular multiplications. In these times of surveillance and extensive profiling there is a great need for an anonymous communication system that resists global attackers. One widely recognized solution to the challenge of traffic analysis is a mixnet, which anonymizes a batch of messages by sending the batch through a fixed cascade of mixnodes. Mixnets can offer excellent privacy guarantees, including unlinkability of sender and receiver, and resistance to many traffic-analysis attacks that undermine many other approaches including onion routing. Existing mixnet designs, however, suffer from high latency in part because of the need for real-time public-key operations. Precomputation greatly improves the real-time performance of cMix, while its fixed cascade of mixnodes yields the strong anonymity guarantees of mixnets. cMix is unique in not requiring any real-time public-key operations by users. Consequently, cMix is the first mixing suitable for low latency chat for lightweight devices. Our presentation includes a specification of cMix, security arguments, anonymity analysis, and a performance comparison with selected other approaches. We also give benchmarks from our prototype

Minimising anonymity loss in anonymity networks under DoS attacks

Anonymity is a security property of paramount importance as it helps to protect users’ privacy by ensuring that their identity remains unknown. Anonymity protocols generally suffer from denial of service (DoS) attack, as repeated message retransmission affords more opportunities for attackers to analyse traffic and lower the protocols’ privacy. In this paper, we analyse how users can minimise their anonymity loss under DoS attacks by choosing to remove or keep ‘failed’ nodes from router lists. We also investigate the strategy effectiveness in those cases where users cannot decide whether the ‘failed’ node are the targets of DoS attacks

Performance measurements of Tor hidden services in low-bandwidth access networks

Abstract. Being able to access and provide Internet services anonymously is an important mechanism to ensure freedom of speech in vast parts of the world. Offering location-hidden services on the Internet requires complex redirection protocols to obscure the locations and identities of communication partners. The anonymity system Tor supports such a protocol for providing and accessing TCP-based services anonymously. The complexity of the hidden service protocol results in significantly higher response times which is, however, a crucial barrier to user acceptance. This communication overhead becomes even more evident when using limited access networks like cellular phone networks. We provide comprehensive measurements and statistical analysis of the bootstrapping of client processes and different sub-steps of the Tor hidden service protocol under the influence of limited access networks. Thereby, we are able to identify bottlenecks for low-bandwidth access networks and to suggest improvements regarding these networks.

Tor HTTP Usage and Information Leakage

International audienceThis paper analyzes the web browsing behaviour of Tor users. By collecting HTTP requests we show which websites are of interest to Tor users and we determined an upper bound on how vulnerable Tor users are to sophisticated de-anonymization attacks: up to 78 % of the Tor users do not use Tor as suggested by the Tor community, namely to browse the web with TorButton. They could thus fall victim to de-anonymization attacks by merely browsing the web. Around 1% of the requests could be used by an adversary for exploit piggybacking on vulnerable file formats. Another 7 % of all requests were generated by social networking sites which leak plenty of sensitive and identifying information. Due to the design of HTTP and Tor, we argue that HTTPS is currently the only effective countermeasure against de-anonymization and information leakage for HTTP over Tor

Cryptographic Protocol Analysis of AN.ON

This work presents a cryptographic analysis of AN.ON’s anonymization protocols. We have discovered three flaws of differing severity. The first is caused by the fact that the freshness of the session key was not checked by the mix. This flaw leads to a situation where an external attacker is able to perform a replay attack against AN.ON. A second, more severe, error was found in the encryption scheme of AN.ON. An internal attacker controlling the first mix in a cascade of length two is able to de-anonymize users with high probability. The third flaw results from the lack of checks to ensure that a message belongs to the current session. This enables an attacker to impersonate the last mix in a cascade. The flaws we discovered represent errors that, unfortunately, still occur quite often and show the importance of either using standardized crytpographic protocols or performing detailed security analyses

DefenestraTor: Throwing out Windows in Tor

Abstract. Tor is the most widely used privacy enhancing technology for achieving online anonymity and resisting censorship. While conventional wisdom dictates that the level of anonymity offered by Tor increases as its user base grows, the most significant obstacle to Tor adoption continues to be its slow performance. We seek to enhance Tor’s performance by offering techniques to control congestion and improve flow control, thereby reducing unnecessary delays. To reduce congestion, we first evaluate small fixed-size circuit windows and a dynamic circuit window that adaptively resizes in response to perceived congestion. While these solutions improve web page response times and require modification only to exit routers, they generally offer poor flow control and slower downloads relative to Tor’s current design. To improve flow control while reducing congestion, we implement N23, an ATM-style per-link algorithm that allows Tor routers to explicitly cap their queue lengths and signal congestion via back-pressure. Our results show that N23 offers better congestion and flow control, resulting in improved web page response times and faster page loads compared to Tor’s current design and the other window-based approaches. We also argue that our proposals do not enable any new attacks on Tor users ’ privacy.

BackRef: Accountability in Anonymous Communication Networks

Many anonymous communication networks (ACNs) rely on routing traffic through a sequence of proxy nodes to obfuscate the originator of the traffic. Without an accountability mechanism, exit proxy nodes may become embroiled in a criminal investigation if originators commit criminal actions through the ACN. We present BackRef, a generic mechanism for ACNs that provides practical repudiation for the proxy nodes by tracing back the selected outbound traffic to the predecessor node (but not in the forward direction) through a cryptographically verifiable chain. It also provides an option for full (or partial) traceability back to the entry node or even to the corresponding originator when all intermediate nodes are cooperating. Moreover, to maintain a good balance between anonymity and accountability, the protocol incorporates whitelist directories at exit proxy nodes. BackRef offers improved deployability over the related work, and introduces a novel concept of pseudonymous signatures that may be of independent interest. We exemplify the utility of BackRef by integrating it into the onion routing (OR) protocol, and examine its deployability by considering several system-level aspects. We also present the security definitions for the BackRef system (namely, anonymity, backward traceability, no forward traceability, and no false accusation) and conduct a formal security analysis of the OR protocol with BackRef using ProVerif, an automated cryptographic protocol verifier, establishing the aforementioned security properties against a strong adversarial model

Valet services: Improving hidden servers with a personal touch

Abstract. Location hidden services have received increasing attention as a means to resist censorship and protect the identity of service operators. Research and vulnerability analysis to date has mainly focused on how to locate the hidden service. But while the hiding techniques have improved, almost no progress has been made in increasing the resistance against DoS attacks directly or indirectly on hidden services. In this paper we suggest improvements that should be easy to adopt within the existing hidden service design, improvements that will both reduce vulnerability to DoS attacks and add QoS as a service option. In addition we show how to hide not just the location but the existence of the hidden service from everyone but the users knowing its service address. Not even the public directory servers will know how a private hidden service can be contacted, or know it exists.