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Computationally Secure Two-Round Authenticated Message Exchange

Klaas Ole Kürtz, Henning Schnoor, and Thomas Wilke

We study two-round authenticated message exchange protocols consisting of a single request and a single response, with the realistic assumption that the responder is long-lived and has bounded memory. We first argue that such protocols necessarily need elements such as timestamps to be secure. We then present such a protocol and prove that it is correct and computationally secure. In our model, the adversary provides the initiator and the responder with the payload of their messages, which means our protocol can be used to implement securely any service based on authenticated message exchange. We even allow the adversary to read and reset the memory of the principals and to use, with very few restrictions, the private keys of the principals for signing the payloads or parts thereof. The latter corresponds to situations in which the keys are not only used by our protocol. We use timestamps to secure our protocol, but only assume that each principal has access to a local clock.