@misc{cryptoeprint:2007:177,
author = {Dan Boneh and Craig Gentry and Michael Hamburg},
title = {Space-Efficient Identity Based Encryption Without Pairings},
howpublished = {Cryptology ePrint Archive, Report 2007/177},
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.