@misc{cryptoeprint:2007:265,
author = {Jonathan Katz},
title = {Which Languages Have 4-Round Zero-Knowledge Proofs?},
howpublished = {Cryptology ePrint Archive, Report 2007/265},
year = {2007},
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.