@misc{cryptoeprint:2008:389,
author = {Murat Ak and Kamer Kaya and Ali Aydin Selcuk},
title = {Optimal Subset-Difference Broadcast Encryption with Free Riders},
howpublished = {Cryptology ePrint Archive, Report 2008/389},
year = {2008},
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.