@misc{cryptoeprint:2003:184,
author = {Nicolas T. Courtois and Guilhem Castagnos and Louis Goubin},
title = {What do DES S-boxes Say to Each Other ?},
howpublished = {Cryptology ePrint Archive, Report 2003/184},
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.