@misc{cryptoeprint:2006:188,
author = {Tetsu Iwata},
title = {New Blockcipher Modes of Operation with Beyond the Birthday Bound Security},
howpublished = {Cryptology ePrint Archive, Report 2006/188},
year = {2006},
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.