@misc{cryptoeprint:2014:236,
author = {Shazia Afreen},
title = {Linear Sequential Circuit Approximation of Acterbahn Stream Cipher},
howpublished = {Cryptology ePrint Archive, Report 2014/236},
year = {2014},
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.