From: andr0id@midwest.net (Jason Rentz)
Date: Thu, 21 Dec 95 12:25:30 PST
To: cypherpunks@toad.com
Subject: Re: 900mhz digital phones - how much to trust ?
Message-ID: <199512212046.OAA18997@cdale1.midwest.net>
MIME-Version: 1.0
Content-Type: text/plain


>
>Whats the current thinking on the security level of 900Mhz digital spread
>sectrum cordless phones? Clearly it's not a basic scanner job but how much
>more equipment is needed to monitor one ?

Well when you listen into a spread spectrum conversation what you will hear
is open squelch white noise.  The spread spectrum radios that I know about
send information on several deffrent frequencies throughout the
conversation.  Unless you have a system to receive the encoding bit that
signal what frequency is next you can't easily find the next freqency before
it changes again.  This change happens several time a second.  The
information is "packet-like", that is why you are able to use so many spread
spectrum units at once.  Example, Lynx spread spectrum T1 information signal
(data) is combined with a high rate spreading code (chip sequence). A
multiplier performs the combination.  Because the spreading code is
pseudo-randomly generated, the combined signal occupies a significantly
expanded bandwidth with a lower uniform power density.  At the receiver, a
locally generated, synchronized replica of the spreading code recovers the
information signal, through a second multiplication.  The same code sequence
must be used in the transmitter and receiver to avoic losing the information. 

The coding is direct sequence, 16 x spreading rate, the number of codes is a
9 DIP switch selectable.

Intresting is taht the radio acquisition time is 500 msec, typical.  If this
a security hole I don't know.

Note this is typical of a Spread Spectrum Microwave radio, a lot of the same
applies to 900MHz T1 and cordless phones.  As soon as I have further info on
encryption of signal/Freq. destination I'll post it.

                Dr0id


( Computer Consulting & Management   )
(P.O. Box 421  Cambria, IL 62915-0421)

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