Xref: utzoo sci.electronics:8280 sci.crypt:2402 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!tut.cis.ohio-state.edu!pt.cs.cmu.edu!unh.cs.cmu.edu!agn From: agn@unh.cs.cmu.edu (Andreas Nowatzyk) Newsgroups: sci.electronics,sci.crypt Subject: Re: Telephone privacy gadgets Add: Cryptography Message-ID: <6596@pt.cs.cmu.edu> Date: 19 Oct 89 22:31:56 GMT References: <799@mccall.uucp> <776@ariel.unm.edu> <790@ariel.unm.edu> <1989Oct19.154929.19256@utzoo.uucp> Organization: Carnegie-Mellon University, CS/RI Lines: 24 >>Expensive and easy to defeat -- probably... > >Most any non-digital scrambler, no matter how tricky, is probably going to >fall under this heading. There is lots and *lots* of redundancy in the >human voice, and it's very hard to hide it completely. Not necessarily true: the (west) german police uses a system that digitizes speech, cuts the data in short blocks, permutes the order of these blocks according to a digitally computed key, converts it back to analog and sends it. A key tone is added to synchronize the receiver so that it can undue the process. The system is not easy to defeat and has the virtue of using no more bandwidth than the original signal. This point is important to be able to use it over existing gear (radios, phone lines, etc.). While a full digital system is possible in the same bandwidth, the complexity is much higher and more likely to break down under marginal signal conditions, interference, and routine day-to-day use. -- Andreas -- -- Andreas Nowatzyk (DC5ZV) Carnegie-Mellon University agn@unh.cs.cmu.edu Computer Science Department (412) 268-3617