If mutually mistrustful parties A and B control two or more appropriatelylocated sites, special relativity can be used to guarantee that a pair ofmessages exchanged by A and B are independent. In earlier work, we used thisfact to define a relativistic bit commitment protocol, RBC1, in which securityis maintained by exchanging a sequence of messages whose transmission rateincreases exponentially in time. We define here a new relativistic protocol,RBC2, which requires only a constant transmission rate and could be practicallyimplemented. We prove that RBC2 allows a bit commitment to be indefinitelymaintained with unconditional security against all classical attacks. Weexamine its security against quantum attacks, and show that it is immune fromthe class of attacks shown by Mayers and Lo-Chau to render non-relativisticquantum bit commitment protocols insecure.
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