@misc{cryptoeprint:2011:522,
author = {Imran Abbasi, Mehreen Afzal},
title = {A Compact S-Box Design for SMS4 Block Cipher},
howpublished = {Cryptology ePrint Archive, Report 2011/522},
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.