@misc{cryptoeprint:2005:400,
author = {Yu Sasaki and Yusuke Naito and Noboru Kunihiro and Kazuo Ohta},
title = {Improved Collision Attack on MD5},
howpublished = {Cryptology ePrint Archive, Report 2005/400},
year = {2005},
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.