@misc{cryptoeprint:2003:253,
author = {Roberto Maria Avanzi},
title = {Aspects of Hyperelliptic Curves over Large Prime Fields in Software Implementations},
howpublished = {Cryptology ePrint Archive, Report 2003/253},
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.