77 lines
4.3 KiB
Plaintext
77 lines
4.3 KiB
Plaintext
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Your Personal Phone Security
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Written By King Lucifer (Alias Byteman) on 8/10/88
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A Telecom Computer Security Bulletin File
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_______________________________________________________________________________
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To secure speech transmissions against evaesdroppers, you could scramble the
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speech information by rearranging its frequency spectrum. This Technique has
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particular application for radio-link transmissions for example, in land
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mobile radios, cellular radios, or cordless phones-in which it's relatively
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easy for third parties to tune in to the transmission frequency.
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To My knowledge, the FX204 variable-split-band Frequency-inverter IC
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performs scrambling or descrambling of speech on one chip. The IC splits
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voiceband information into High and Low Frequency Bands and then inverts
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each band around its center frequency. The FX204's switched capacitor
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filters split the frequency spectrum, and its balanced modulators perform
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the frequency inversion of each band. All Clocking information for the filters
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and modulators comes from a single external 1-MHz crystal that you can connect
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directly across two of the IC's pins.
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Five CMOS-Compatible latched programming inputs allow you to program the
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device's internal clock dividers to produce one of 32 diffrent voiceband-
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frequency split points between 350Hz and 2800Hz. You can hard-wire these
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inputs to produce a fixed-code scrambler, or you can Dynamically change them
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during the transmission to produce a rolling-code scrambler.
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The IC's Transmit/Recive control input optimizes the internal operation
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of the FX204 for scrambling or descrambling. To recombine the upper and lower
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frequency bands and provide a low-impedance audio output, the IC has on-chip
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summing amplifier with external gain defining resistors.
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The FX204 requires a transmission-channel bandwidth of 300 to 3400 Hz;
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However it reduces the Upper bandwidth limit of the transmitted speech
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frequency to 2700 Hz. The Redirection in effective speech-frequency bandwidth
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occurs because the fx204 separates the high and low frquency band by 700Hz in
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order to reduce spurious outputs.
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You can opeate the device over narrower transmission-channel bandwidths
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for example, the 3-kHz bandwidth used for mobile radio by reducing your
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external crystal oscillator's clock frequency. The Audio Input is capable
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of handling a maximum input signal of at least 340 m V rms; the Dynamic
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range is 25 dB typ.
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Fabricated in CMOS technology, the FX204 operates from a single 5v supply.
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It draws 3 mA typ (10 mA max) of supply current so it's suitable for use
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in battery-powered equipment. Supplied in a 24-pin ceramic DIP or Surface-
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Mount Flat pack...
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Schematics
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----------
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VDD__.__. _. \._ _. \._ __.__
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| |... |__ | |__ | ___((_|
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| (((( \\\ | \\\ | _!_((. |
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| |||| Clear | Rx/Tx|________|___|_|.___
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__________|__....________|_______|________|___|_|____|
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| |__|-------| | | | | | |
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_____|______ |__|--Rom--| | |________|___|_|____|
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| | | -)] |
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| | * \/ | |
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|____|_______|____{ - } | | (Audio Out)
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| | | | | | |____/-\_/\_
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|_________________[ + ]______|_________[ + ]_________._____|____( )__ |
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| | ______________|____.___.| |
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| | 0 0 [_(>-----------|----||--\/---+
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|____|___|(300 2700 Audio Input) ______________|_____|___|___|
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|____|_____________[ + ]_____ ______ _[ + ]____________________|__( )--+
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--------- || || |
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| FX204 | || || |
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--------- (Pin 1) (Pin 2)__\/______\/__________________________|
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_______________________________________________________________________________
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$
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