@misc{cryptoeprint:2007:206,
author = {Pierre-Alain Fouque and Gaetan Leurent and Phong Nguyen},
title = {Automatic Search of Differential Path in MD4},
howpublished = {Cryptology ePrint Archive, Report 2007/206},
year = {2007},
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.