Many applications of cryptographic identification protocols are vulnerable against physical adversaries who perform real time attacks. For instance, when identifying a physical object like an automated teller machine, common identification schemes can be bypassed by faithfully relaying all messages between the communicating participants. This attack is known as mafia fraud. The Probabilistic Channel Hopping (PCH) system we introduce in this paper, solves this problem by hiding the conversation

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... hannel between the participants. The security of our approach is based on the assumption that an adversary cannot efficiently relay all possible communication channels of the PCH system in parallel.