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- Newsgroups: sci.crypt
- Path: sparky!uunet!think.com!ames!sgi!rigden.wpd.sgi.com!rpw3
- From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
- Subject: Re: Motorola 'Secure-Clear' Cordless Telephones
- Message-ID: <ueuhkm0@sgi.sgi.com>
- Sender: rpw3@rigden.wpd.sgi.com
- Organization: Silicon Graphics, Inc. Mountain View, CA
- Date: Mon, 4 Jan 1993 12:30:32 GMT
- Lines: 53
-
- In article <C05JAM.MJL@ais.org> tim@ais.org (Tim Tyler) writes:
- +---------------
- | For those of you not familiar with speech inversion, it simply
- | flip-flops the voice spectrum so that high pitched sounds are low,
- | & vice versa. It sounds a lot like Single Side Band (SSB)
- | transmissions, although an SSB receiver will not decode speech-
- | inversion scrambling.
- +---------------
-
- As a general statement, incorrect. If the sending radio uses AM, DSB, or SSB
- modulation, it's trivial. All you have to do is listen to the upper sideband
- of the speech-inverted signal as if it were a *lower*-sideband signal! Or
- vice-versa, it doesn't matter. (You will of course have to manually search
- for the inversion frequency.)
-
- What you *may* have meant is that inverted speech sent via *FM* (such as
- 49 MHz cordless phones) cannot (easily) be received with an unassisted SSB
- receiver. This is more nearly true, but still not a great protection:
-
- 1. Many cheap FM transmitters often have a significant AM component,
- sometimes even enough for an SSB receiver to be used as above
- (though with considerable other "garbage" leaking through).
-
- 2. If the FM "deviation" is large, there will be significant AM-like sidebands
- that might be listened to with an SBB receiver with sufficiently narrow
- filters. [O.k., my math's weak on this one. Maybe Phil Karn or some other
- radio-knowledgable type could comment further on this case.]
-
- 3. There also may be other "spurious emissions" from the FM set that a good
- AM or SSB set could snoop [such as amplitude variations of an oscillator
- due to poor power-supply filtering between the audio & RF sections of the
- phone]. Just because something meets FCC "Part 15" doesn't mean that it's
- of "TEMPEST" quality...
-
- 4. Finally, one simple way to decode speech-inverted FM is to send the audio
- output of an FM receiver (scanner) into an ordinary AM (or SSB) transmitter
- (such as an AM "wireless mike"), and then listen to the output of *that*
- with your SSB receiver on the "wrong" sideband. [This is in fact sort of
- how speech-inversion receivers work internally, using a 10's to 100's of
- KHz internal "AM" channel, and "SSB" reception of the "wrong" sideband by
- filtering (then "carrier-injection", then "AM reception").]
-
- In any case, we all agree that speech-inversion is hardly secure.
-
-
- -Rob
-
- -----
- Rob Warnock, MS-9U/510 rpw3@sgi.com
- Silicon Graphics, Inc. (415)390-1673
- 2011 N. Shoreline Blvd.
- Mountain View, CA 94043
-
-