@misc{cryptoeprint:2003:020,
author = {Bernhard L\"ohlein},
title = {Attacks based on Conditional Correlations against the Nonlinear Filter Generator},
howpublished = {Cryptology ePrint Archive, Report 2003/020},
year = {2003},
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.