@misc{cryptoeprint:2004:180,
author = {Brent R. Waters},
title = {Efficient Identity-Based Encryption Without Random Oracles},
howpublished = {Cryptology ePrint Archive, Report 2004/180},
year = {2004},
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.