@misc{cryptoeprint:2000:033,
author = {Kazumaro Aoki},
title = {The Complete Distribution of Linear Probabilities of MARS' s-box},
howpublished = {Cryptology ePrint Archive, Report 2000/033},
year = {2000},
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.