@misc{cryptoeprint:2002:121,
author = {Tanja Lange},
title = {Efficient Arithmetic on Genus 2 Hyperelliptic Curves over Finite Fields via Explicit Formulae},
howpublished = {Cryptology ePrint Archive, Report 2002/121},
year = {2002},
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.