@misc{cryptoeprint:2011:218,
author = {Markku-Juhani O. Saarinen},
title = {Cryptographic Analysis of All 4 x 4 - Bit S-Boxes },
howpublished = {Cryptology ePrint Archive, Report 2011/218},
year = {2011},
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.