@misc{cryptoeprint:2001:106,
author = {Steve Babbage},
title = {Cryptanalysis of the COS (2,128) Stream Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2001/106},
year = {2001},
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.