@misc{cryptoeprint:2009:501,
author = {Joppe W. Bos, Dag Arne Osvik, and Deian Stefan},
title = {Fast Implementations of AES on Various Platforms},
howpublished = {Cryptology ePrint Archive, Report 2009/501},
year = {2009},
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.