@misc{cryptoeprint:2011:624,
author = {Itai Dinur and Orr Dunkelman and Adi Shamir},
title = {New attacks on Keccak-224 and Keccak-256},
howpublished = {Cryptology ePrint Archive, Report 2011/624},
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.