@misc{cryptoeprint:2012:668,
author = {Gaetan Leurent},
title = {Construction of Differential Characteristics in ARX Designs -- Application to Skein},
howpublished = {Cryptology ePrint Archive, Report 2012/668},
year = {2012},
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.