@misc{cryptoeprint:2015:131,
author = {Frederik Armknecht, Vasily Mikhalev},
title = {On Lightweight Stream Ciphers with Shorter Internal States},
howpublished = {Cryptology ePrint Archive, Report 2015/131},
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.