@misc{cryptoeprint:2007:057,
author = {David Freeman},
title = {Constructing pairing-friendly genus 2 curves over prime fields with ordinary Jacobians},
howpublished = {Cryptology ePrint Archive, Report 2007/057},
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.