@misc{cryptoeprint:2010:643,
author = {Tao Xie and Dengguo Feng},
title = {Construct MD5 Collisions Using Just A Single Block Of Message},
howpublished = {Cryptology ePrint Archive, Report 2010/643},
year = {2010},
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.