@misc{cryptoeprint:2011:295,
author = {P. Gaudry and D. Kohel and B. Smith},
title = {Counting Points on Genus 2 Curves with Real Multiplication},
howpublished = {Cryptology ePrint Archive, Report 2011/295},
year = {2011},
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.