@misc{cryptoeprint:2003:064,
author = {Pascal Junod},
title = {On the Optimality of Linear, Differential and Sequential Distinguishers},
howpublished = {Cryptology ePrint Archive, Report 2003/064},
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.