Stop-and-Go-MIXes Providing Probabilistic Anonymity in an Open System

Currently known basic anonymity techniques depend on identity veri cation. If veri cation of user identities is not possible due to the related management overhead or a general lack of information (e.g. on the Internet), an adversary can participate several times in a communication relationship and observe the honest users. In this paper we focus on the problem of providing anonymity without identity veri cation. The notion of probabilistic anonymity is introduced. Probabilistic anonymity is based on a publicly known security parameter, which determines the security of the protocol. For probabilistic anonymity the insecurity, expressed as the probability of having only one honest participant, approaches 0 at an exponential rate as the security parameter is changed linearly. Based on our security model we propose a new MIX variant called "Stop-and-Go-MIX" (SG-MIX) which provides anonymity without identity veri cation, and prove that it is probabilistically secure.