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Abstract: We propose a general security definition for cryptographic quantum protocolsthat implement classical non-reactive two-party tasks. The definition isexpressed in terms of simple quantum-information-theoretic conditions whichmust be satisfied by the protocol to be secure. The conditions are uniquelydetermined by the ideal functionality F defining the cryptographic task to beimplemented. We then show the following composition result. If quantumprotocols pi 1,

.,pi k securely implement ideal functionalities F 1,

.,F kaccording to our security definition, then any purely classical two-partyprotocol, which makes sequential calls to F 1,

.,F k, is equally secure as theprotocol obtained by replacing the calls to F 1,

.,F k with the respectivequantum protocols pi 1,

.,pi k. Hence, our approach yields the minimalsecurity requirements which are strong enough for the typical use of quantumprotocols as subroutines within larger classical schemes. Finally, we show thatrecently proposed quantum protocols for oblivious transfer and secureidentification in the bounded-quantum-storage model satisfy our securitydefinition, and thus compose in the above sense.

Author: Serge Fehr, Christian Schaffner


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