We present Tor, a circuit-based low-latency anonymous communication service. This second-generation Onion Routing system addresses limitations in the original design by adding perfect forward secrecy, congestion control, directory servers, integrity checking, configurable exit policies, and a practical design for location-hidden services via rendezvous points. Tor works on the real-world Internet, requires no special privileges or kernel modifications, requires little synchronization or coordination between nodes, and provides a reasonable tradeoff between anonymity, usability, and efficiency. We briefly describe our experiences with an international network of more than 30 nodes. We close with a list of open problems in anonymous communication. OverviewOnion Routing is a distributed overlay network designed to anonymize TCP-based applications like web browsing, secure shell, and instant messaging. Clients choose a path through the network and build a circuit, in which each node (or "onion router" or "OR") in the path knows its predecessor and successor, but no other nodes in the circuit. Traffic flows down the circuit in fixed-size cells, which are unwrapped by a symmetric key at each node (like the layers of an onion) and relayed downstream. The Onion Routing project published several design and analysis papers [27,41,48,49]. While a wide area Onion Routing network was deployed briefly, the only long-running public implementation was a fragile proofof-concept that ran on a single machine. Even this simple deployment processed connections from over sixty thousand distinct IP addresses from all over the world at a rate of about fifty thousand per day. But many critical design and deployment issues were never resolved, and the design has not been updated in years. Here we describe Tor, a protocol for asynchronous, loosely federated onion routers that provides the following improvements over the old Onion Routing design:Perfect forward secrecy: In the original Onion Routing design, a single hostile node could record traffic and later compromise successive nodes in the circuit and force them to decrypt it. Rather than using a single multiply encrypted data structure (an onion) to lay each circuit, Tor now uses an incremental or telescoping path-building design, where the initiator negotiates session keys with each successive hop in the circuit. Once these keys are deleted, subsequently compromised nodes cannot decrypt old traffic. As a side benefit, onion replay detection is no longer necessary, and the process of building circuits is more reliable, since the initiator knows when a hop fails and can then try extending to a new node.Separation of "protocol cleaning" from anonymity: Onion Routing originally required a separate "application proxy" for each supported application protocol-most of which were never written, so many applications were never supported.Tor uses the standard and near-ubiquitous SOCKS [32] proxy interface, allowing us to support most TCP-based programs without modification. Tor now reli...

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Abstract. We extend earlier research on mounting and resisting passive long-term end-to-end traffic analysis attacks against anonymous message systems, by describing how an eavesdropper can learn sender-receiver connections even when the substrate is a network of pool mixes, the attacker is non-global, and senders have complex behavior including generating padding messages. Additionally, we describe how an attacker can use extra information about message distinguishability to speed the attack. Finally, we simulate our attacks for a variety of scenarios, focusing on the amount of information needed to link senders to their recipients. In each scenario, we show that the intersection attack can still succeed, albeit more slowly-in some cases, so slowly as to be impractical.

Today's software update systems have little or no defense against key compromise. As a result, key compromises have put millions of software update clients at risk. Here we identify three classes of information whose authenticity and integrity are critical for secure software updates. Analyzing existing software update systems with our framework, we find their ability to communicate this information securely in the event of a key compromise to be weak or nonexistent. We also find that the security problems in current software update systems are compounded by inadequate trust revocation mechanisms. We identify core security principles that allow software update systems to survive key compromise. Using these ideas, we design and implement TUF, a software update framework that increases resilience to key compromise.

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