@misc{cryptoeprint:2015:285,
author = {Tomer Ashur},
title = {Improved Linear Trails for the Block Cipher Simon},
howpublished = {Cryptology ePrint Archive, Report 2015/285},
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.