@misc{cryptoeprint:2011:332,
author = {Joan Boyar and Rene Peralta},
title = {A depth-16 circuit for the AES S-box},
howpublished = {Cryptology ePrint Archive, Report 2011/332},
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.