@misc{cryptoeprint:2003:251,
author = {EunKyung Jeong},
title = {Isomorphism Classes of Hyperelliptic Curves of genus 3 over finite fields},
howpublished = {Cryptology ePrint Archive, Report 2003/251},
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.