@misc{cryptoeprint:2008:147,
author = {Timo Gendrullis and Martin Novotny and Andy Rupp},
title = {A Real-World Attack Breaking A5/1 within Hours},
howpublished = {Cryptology ePrint Archive, Report 2008/147},
year = {2008},
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.