@misc{cryptoeprint:2005:314,
author = {P. Gaudry},
title = {Fast genus 2 arithmetic based on Theta functions},
howpublished = {Cryptology ePrint Archive, Report 2005/314},
year = {2005},
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.