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There are different methods to build an anonymity service using MIXes. A substantial decision for doing so is the method of choosing the MIX route. In this paper we compare two special configurations: a fixed MIX route used by all participants and a network of freely usable MIXes where each participant chooses his own route. The advantages and disadvantages in respect to the freedom of choice are presented and examined. We'll show that some additional attacks are possible in networks with freely chosen MIX routes. After describing these attacks, we estimate their impact on the achievable degree of anonymity. Finally, we evaluate the relevance of the described attacks with respect to existing systems like e.g. Mixmaster, Crowds, and Freedom.

Abstract. It is a hard problem to achieve anonymity for real-time services in the Internet (e.g. Web access). All existing concepts fail when we assume a very strong attacker model (i.e. an attacker is able to observe all communication links). We also show that these attacks are realworld attacks. This paper outlines alternative models which mostly render these attacks useless. Our present work tries to increase the efficiency of these measures. 1ÊÊThe perfect system 1.1ÊÊ AttacksThe perfect anonymous communication system has to prevent the following attacks:1. Message coding attack: If messages do not change their coding during transmission they can be linked or traced. 2. Timing attack: An opponent can observe the duration of a specific communication by linking its possible endpoints and waiting for a correlation between the creation and/or release event at each possible endpoint. 3. Message volume attack: The amount of transmitted data (i.e. the message length) can be observed. Thus, a global observer is able to associate a communication relation to a certain client and server. 4. Flooding attack: Each message can only be anonymous in a group of sent messages. All senders of those messages form the anonymity group. Under normal circumstances, each sender should send one message per time interval. However, some of the existing concepts enable an attacker to flood the system in order to separate a certain message. 5. Intersection attack: Because of the on-line/off-line periods of the users an attacker may trace them by observation over a long period. 6. Collusion attack: A corrupt coalition of users or parts of the system may be able to trace certain users. Perfect means that there cannot occur a situation where an opponent gets valuable information concerning any communication relation or communication request from and to a certain user. However, no system can protect from an opponent with unlimited power. Therefore we assume that the opponent may not be able to break into cryptographic functions. Though, we have to consider that parts of the anonymous communication system may act as opponents (insider attacks). 1.2ÊÊ Functions of a perfect systemPrevention of collusion attack. A perfect anonymous communication system will be a distributed system. No central system can protect from a corrupt insider since he has all information concerning the sender and recipient of a communication relation. Thus, he can observe a commu-

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