Quantum Oblivious Mutual Identification

We coiisider a situation where two parties, Alice and Bob, share a common secret string arid would like to mutually check their knowledge of that string. We describe a simple and efficient protocol based on the exchange of quantum information to check mutual knowledge of a common string in such a way that honest parties will always succeed in convincing each other, while a dishonest party interacting with an honest party will have vanishingly small probability of convincing him. Moieover, a dishonest party gains only d very small amount, of information about t,he secret string from running the protocol: whoever enters the protocol with no knowledge of the secret string would have to enter this protocol an exponential number of times in order to gain non-negligible information about the string. Our scheme offers an efficient identification technique with a security that depends on no computational assumption, only on the correctness of quantum mechanics. We believe such a system should be used in smartcards to avoid frauds from typing PIN codes to dishonest teller machines. 1 In t 1-0 d u c t i o 11 Weren’t you worried tlie last time you typed your PIN (Personal Identification Number) to an unknown teller machine that it could be a fake and that its sole purpose could he to steal your PIN? According to a recent headline of the NY Times [as] maybe YOU should worry: “ONE LESS THING TO BELIEVE IN: FRAUD AT FAKE CASH MACHINE” The problem with current identification systems is that the customer must trust tlie equipment to which he types his PIN. It is completely trivial to modify a teller machine to memorize the PIN numbers that people type to it. PINS are meant to be checked, not given. (Consult [I] for an extensive study of frauds at teller machines.) Of course, it is always possible to completely solve the problem of identification and authentication of messages by classical methods [9] that require L.C. Guillou and J.-J. Quisquater (Eds.): Advances in Cryptology EUROCRYPT ’95, LNCS 921, pp. 133-146, 1995. 0 Spnnger-Verlag Berlin Heidelberg 1995