Xref: utzoo sci.electronics:8324 sci.crypt:2422 Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!apple!vsi1!daver!lynx!neal From: neal@lynx.uucp (Neal Woodall) Newsgroups: sci.electronics,sci.crypt Subject: Re: Telephone privacy gadgets Add: Cryptography Summary: Privacy using "rolling codes" and split band inversion.... Message-ID: <6422@lynx.UUCP> Date: 24 Oct 89 23:01:30 GMT References: <799@mccall.uucp> <776@ariel.unm.edu> <790@ariel.unm.edu> <1989Oct19.154929.19256@utzoo.uucp> <6596@pt.cs.cmu.edu> Reply-To: neal@lynx.UUCP (Neal Woodall) Distribution: usa Organization: Lynx Real-Time Systems Inc, Campbell CA Lines: 42 In article <6596@pt.cs.cmu.edu> Andreas Nowatzyk writes: >>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. I have heard of systems like this, and I think that some government agencies (maybe the Secret Service) tried them for a while. The Secret Service now uses Motorola's DES system. Another system that is available on the open market here in the US is the so-called "split band rolling code" scrambler. It is a combination of digital and analog technologies, and offers reasonable voice security at a decent price. These systems are designed to be used on common commercial two-way FM radios. Basically, the system spilts the voice band into two pieces (upper and lower)....there are 32 possible split-points. When the voice band has been split, each piece of the band is inverted, then the two inverted pieces are recombined and transmitted over the radio channel. The neat trick is that the split point in the voice band is changed from 4 to 60 times per second, controlled by a "rolling code", which, from what I can gather, is a pseudo-random generator.....the seed that you choose determines the code sequence that splits the voice band. To maintain sync, the system transmits a sync-burst every second or so....this allows people to join a conversation in progress (if they have the key!). A very robust error correction system ensures that the sync burst gets through noise. Integrated circuit designed for the various functions needed are made by MX-COM in Winston-Salem, NC. If you are so inclined, I feel certain that some of the IC's available could be adapted to use in a phone scrambler. Neal