@misc{cryptoeprint:2007:010,
author = {David Freeman and Kristin Lauter},
title = {Computing endomorphism rings of Jacobians of genus 2 curves over finite fields},
howpublished = {Cryptology ePrint Archive, Report 2007/010},
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.