An authentication counterpart of Wyner's study of the wiretap channel is developed in this work. More specifically, message authentication over noisy channels is studied while impersonation and substitution attacks are investigated for both singleand multiple-message scenarios. For each scenario, informationtheoretic lower and upper bounds on the opponent's success, or cheating, probability are derived. Remarkably, in both scenarios, the lower and upper bounds are shown to match, and hence, the

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... fundamental limits on message authentication over noisy channels are fully characterized. The opponent's success probability is further shown to be smaller than that derived in the classical noiseless channel model. These results rely on a novel authentication scheme in which shared key information is used to provide simultaneous protection against both types of attacks. Finally, message authentication for the case in which the source and receiver possess only correlated sequences is studied.