@misc{cryptoeprint:2005:338,
author = {Lu Xiao and Howard M. Heys},
title = {An Improved Power Analysis Attack Against Camellia's Key Schedule},
howpublished = {Cryptology ePrint Archive, Report 2005/338},
year = {2005},
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.