@misc{cryptoeprint:2009:604,
author = {Eleanor Birrell and Salil Vadhan},
title = {Composition of Zero-Knowledge Proofs with Efficient Provers},
howpublished = {Cryptology ePrint Archive, Report 2009/604},
year = {2009},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.