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.

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Tor: the second-generation onion router

Advances in sensing and tracking technology enable location-based applications but they also create significant privacy risks. Anonymity can provide a high degree of privacy, save service users from dealing with service providers’ privacy policies, and reduce the service providers’ requirements for safeguarding private information. However, guaranteeing anonymous usage of location-based services requires that the precise location information transmitted by a user cannot be easily used to re-identify the subject. This paper presents a middleware architecture and algorithms that can be used by a centralized location broker service. The adaptive algorithms adjust the resolution of location information along spatial or temporal dimensions to meet specified anonymity constraints based on the entities who may be using location services within a given area. Using a model based on automotive traffic counts and cartographic material, we estimate the realistically expected spatial resolution for different anonymity constraints. The median resolution generated by our algorithms is 125 meters. Thus, anonymous location-based requests for urban areas would have the same accuracy currently needed for E-911 services; this would provide sufficient resolution for wayfinding, automated bus routing services and similar location-dependent services.

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Anonymous Usage of Location-Based Services Through Spatial and Temporal Cloaking

In this paper we introduce a system called Crowds for protecting users' anonymity on the world-wide-web. Crowds, named for the notion of “blending into a crowd,” operates by grouping users into a large and geographically diverse group (crowd) that collectively issues requests on behalf of its members. Web servers are unable to learn the true source of a request because it is equally likely to have originated from any member of the crowd, and even collaborating crowd members cannot distinguish the originator of a request from a member who is merely forwarding the request on behalf of another. We describe the design, implementation, security, performance, and scalability of our system. Our security analysis introduces degrees of anonymity as an important tool for describing and proving anonymity properties.

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Crowds: anonymity for Web transactions

We describe Freenet, an adaptive peer-to-peer network application that permits the publication, replication, and retrieval of data while protecting the anonymity of both authors and readers. Freenet operates as a network of identical nodes that collectively pool their storage space to store data files and cooperate to route requests to the most likely physical location of data. No broadcast search or centralized location index is employed. Files are referred to in a location-independent manner, and are dynamically replicated in locations near requestors and deleted from locations where there is no interest. It is infeasible to discover the true origin or destination of a file passing through the network, and difficult for a node operator to determine or be held responsible for the actual physical contents of her own node.

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Freenet: a distributed anonymous information storage and retrieval system

Gigantically comprehensive and carefully researched, Security Engineering makes it clear just how difficult it is to protect information systems from corruption, eavesdropping, unauthorized use, and general malice. Better, Ross Anderson offers a lot of thoughts on how information can be made more secure (though probably not absolutely secure, at least not forever) with the help of both technologies and management strategies. His work makes fascinating reading and will no doubt inspire considerable doubt--fear is probably a better choice of words--in anyone with information to gather, protect, or make decisions about. Be aware: This is absolutely not a book solely about computers, with yet another explanation of Alice and Bob and how they exchange public keys in order to exchange messages in secret. Anderson explores, for example, the ingenious ways in which European truck drivers defeat their vehicles' speed-logging equipment. In another section, he shows how the end of the cold war brought on a decline in defenses against radio-frequency monitoring (radio frequencies can be used to determine, at a distance, what's going on in systems--bank teller machines, say), and how similar technology can be used to reverse-engineer the calculations that go on inside smart cards. In almost 600 pages of riveting detail, Anderson warns us not to be seduced by the latest defensive technologies, never to underestimate human ingenuity, and always use common sense in defending valuables. A terrific read for security professionals and general readers alike. --David Wall Topics covered: How some people go about protecting valuable things (particularly, but not exclusively, information) and how other people go about getting it anyway. Mostly, this takes the form of essays (about, for example, how the U.S. Air Force keeps its nukes out of the wrong hands) and stories (one of which tells of an art thief who defeated the latest technology by hiding in a closet). Sections deal with technologies, policies, psychology, and legal matters.

Security Engineering: A Guide to Building Dependable Distributed Systems

Onion routing is an infrastructure for private communication over a public network. It provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. Onion routing's anonymous connections are bidirectional, near real-time, and can be used anywhere a socket connection can be used. Any identifying information must be in the data stream carried over an anonymous connection. An onion is a data structure that is treated as the destination address by onion routers; thus, it is used to establish an anonymous connection. Onions themselves appear different to each onion router as well as to network observers. The same goes for data carried over the connections they establish. Proxy-aware applications, such as Web browsers and e-mail clients, require no modification to use onion routing, and do so through a series of proxies. A prototype onion routing network is running between our lab and other sites. This paper describes anonymous connections and their implementation using onion routing. This paper also describes several application proxies for onion routing, as well as configurations of onion routing networks.

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Anonymous connections and onion routing

Onion routing provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. Unmodified Internet applications can use these anonymous connections by means of proxies. The proxies may also make communication anonymous by removing identifying information from the data stream. Onion routing has been implemented on Sun Solaris 2.X with proxies for Web browsing, remote logins and e-mail. This paper's contribution is a detailed specification of the implemented onion routing system, a vulnerability analysis based on this specification, and performance results.

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reserving privacy means not only hiding the content of messages, but also hiding who is talking to whom (traffic analysis). Much like a physical envelope, the simple application of cryptography within a packet-switched network hides the contents of messages being sent, but can reveal who is talking to whom, and how often. Onion Routing is a general-purpose infrastructure for private communication over a public network [3, 4, 6]. It provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. The connections are bidirectional, near real-time, and can be used for both connection-based and connectionless traffic. Onion Routing interfaces with off-theshelf application software and systems through specialized proxies, making it easy to integrate into existing systems. Prototypes have been running since July 1997. At press time, the prototype network is processing more than one million Web connections per month from more than six thousand IP addresses in twenty countries and in all six main top level domains. Onion Routing operates by dynamically building anonymous connections within a network of real-time Chaum Onion Routing

Onion routing

This paper describes an architecture, Onion Routing, that limits a network's vulnerability to traffic analysis. The architecture provides anonymous socket connections by means of proxy servers. It provides real-time, bi-directional, anonymous communication for any protocol that can be adapted to use a proxy service. Specifically, the architecture provides for bi-directional communication even though no-one but the initiator's proxy server knows anything but previous and next hops in the communication chain. This implies that neither the respondent nor his proxy server nor any external observer need know the identity of the initiator or his proxy server. A prototype of Onion Routing has been implemented. This prototype works with HTTP (World Wide Web) proxies. In addition, an analogous proxy for TELNET has been implemented. roxies for FTP and SMTP are under development.

Hiding Routing Information

: Preserving privacy means not only hiding the content of messages, but also hiding who is talking to whom (traffic analysis). Much like a physical envelope, the simple application of cryptography within a packet-switched network hides the messages being sent, but can reveal who is talking to whom, and how often. Onion Routing is a general purpose infrastructure for private communication over a public network [8, 9, 4]. It provides anonymous connections that are strongly resistant to both eavesdropping and traffic analysis. The connections are bidirectional, near real-time, and can be used for both connection-based and connectionless traffic. Onion Routing interfaces with off the shelf software and systems through specialized proxies, making it easy to integrate into existing systems. Prototypes have been running since July 1997. As of this article's publication, the prototype network is processing more than 1 million Web connections per month from more than six thousand IP addresses in twenty countries and in all six main top level domains. [7]

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David M. Goldschlag

Papers

Anonymous connections and onion routing

Anonymous connections and onion routing

Onion routing

Hiding Routing Information

Onion Routing for Anonymous and Private Internet Connections