Designing and attacking anonymous communication systems

作者:George Danezis


摘要:This report contributes to the field of anonymous communications over widely deployed communication networks. It describes novel schemes to protect anonymity; it also presents powerful new attacks and new ways of analysing and understanding anonymity properties.

We present Mixminion, a new generation anonymous remailer, and examine its security against all known passive and active cryptographic attacks. We use the secure anonymous replies it provides, to describe a pseudonym server, as an example of the anonymous protocols that mixminion can support. The security of mix systems is then assessed against a compulsion threat model, in which an adversary can request the decryption of material from honest nodes. A new construction, the fs-mix, is presented that makes tracing messages by such an adversary extremely expensive.

Moving beyond the static security of anonymous communication protocols, we define a metric based on information theory that can be used to measure anonymity. The analysis of the pool mix serves as an example of its use. We then create a framework within which we compare the traffic analysis resistance provided by different mix network topologies. A new topology, based on expander graphs, proves to be efficient and secure. The rgb-mix is also presented; this implements a strategy to detect flooding attacks against honest mix nodes and neutralise them by the use of cover traffic.

Finally a set of generic attacks are studied. Statistical disclosure attacks model the whole anonymous system as a black box, and are able to uncover the relationships between long-term correspondents. Stream attacks trace streams of data travelling through anonymizing networks, and uncover the communicating parties very quickly. They both use statistical methods to drastically reduce the anonymity of users. Other minor attacks are described against peer discovery and route reconstruction in anonymous networks, as well as the naïve use of anonymous replies.