@misc{cryptoeprint:2015:367,
author = {Achiya Bar-On},
title = {Improved Higher-Order Differential Attacks on MISTY1},
howpublished = {Cryptology ePrint Archive, Report 2015/367},
year = {2015},
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.