We present Tor, a circuit-based low-latency anonymous communication service. This second-generation Onion Routing system addresses limitations in the original design. Tor adds perfect forward secrecy, congestion control, directory servers, integrity checking, configurable exit policies, and a practical design for 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 a dozen hosts. We close with a list of open problems in anonymous communication. 1. Overview

Abstract. There are many unexpected or unexpectedly difficult obstacles to deploying anonymous communications. Drawing on our experiences deploying Tor (the second-generation onion routing network), we describe social challenges and technical issues that must be faced in building, deploying, and sustaining a scalable, distributed, low-latency anonymity network. 1

Abstract—The benefits of caching for reducing access time to frequently needed data, in order to improve system performance, are already well-known. In this paper, a proposal for employing data caching for increasing the level of anonymity provided by an anonymity system is presented. This technique is especially effective for user sessions containing bidirectional communication, such as anonymous web browsing. A framework is first constructed for capturing the effect of attacks on anonymity systems that have the ability to serve some incoming user requests from their cache. A system-wide metric is then presented for measuring the anonymity provided by such systems. It is shown that the anonymity level of such systems rises with the amount of data caching performed by them. This behavior is illustrated in an example threshold mix network. Index Terms—Anonymity Metrics, Caching, Privacy. I.