  If mutually mistrustful parties A and B control two or more appropriately
located sites, special relativity can be used to guarantee that a pair of
messages exchanged by A and B are independent. In earlier work, we used this
fact to define a relativistic bit commitment protocol, RBC1, in which security
is maintained by exchanging a sequence of messages whose transmission rate
increases exponentially in time. We define here a new relativistic protocol,
RBC2, which requires only a constant transmission rate and could be practically
implemented. We prove that RBC2 allows a bit commitment to be indefinitely
maintained with unconditional security against all classical attacks. We
examine its security against quantum attacks, and show that it is immune from
the class of attacks shown by Mayers and Lo-Chau to render non-relativistic
quantum bit commitment protocols insecure.
