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We present a set of simple techniques for key establishment over a radio link in peer-to-peer networks. Our approach is based on the Diffie-Hellmankey agreement protocol, which is known to be vulnerable to the "man-in-the-middle" attack if the two users involved in the protocol do not share any authenticated information about each other (e.g., public keys, certificates, passwords,shared keys, etc.) prior to the protocol execution. In this paper, we solve the problem by leveraging on the natural ability of users to authenticate each other by visual and verbal contact. We propose three techniques. The first is based on visual comparison of short strings, the second on distance bounding, and the third on integrity codes; in each case, the users do not need to enter any password or other data, nor do they need physical or infrared connectivity between their devices. We base our analysis on a well-established methodology …