@misc{cryptoeprint:2009:540,
author = {David Mandell Freeman},
title = {Converting Pairing-Based Cryptosystems from Composite-Order Groups to Prime-Order Groups},
howpublished = {Cryptology ePrint Archive, Report 2009/540},
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.