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The problem of authentication of mutually suspicious parties is one that is becoming more and more important with the proliferation of distributed systems. In this paper we describe a protocol, based on the difficulty of finding discrete logarithms over finite fields, by which users can verify whether they have matching credentials without revealing their credentials to each other unless there is a match. This protocol requires a trusted third party, but does not require it to be available to the users except when they sign up for the system. Thus it is useful in situations in which a trusted third party exists but is not available to all users at all times.