@misc{cryptoeprint:2013:728,
author = {Robert Wicik and Tomasz Rachwalik},
title = {Modified Alternating Step Generators},
howpublished = {Cryptology ePrint Archive, Report 2013/728},
year = {2013},
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.