@misc{cryptoeprint:2015:145,
author = {Stefan Kölbl and Gregor Leander and Tyge Tiessen},
title = {Observations on the SIMON block cipher family},
howpublished = {Cryptology ePrint Archive, Report 2015/145},
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.