@misc{cryptoeprint:2003:213,
author = {Y.Choie and E.Jeong},
title = {Isomorphism Classes of Hyperelliptic Curves of Genus 2 over $\mathbb{F}_{2^n}$},
howpublished = {Cryptology ePrint Archive, Report 2003/213},
year = {2003},
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.