@misc{cryptoeprint:2005:263,
author = {Jun Yajima and Takeshi Shimoyama},
title = {Wang's sufficient conditions of MD5 are not sufficient},
howpublished = {Cryptology ePrint Archive, Report 2005/263},
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.