Path: utzoo!mnetor!uunet!nbires!hao!gatech!gitpyr!kludge
From: kludge@pyr.gatech.EDU (Scott Dorsey)
Newsgroups: sci.crypt
Subject: Re: One time pads and spy devices
Message-ID: <5101@pyr.gatech.EDU>
Date: 29 Feb 88 21:20:46 GMT
References: <161200004@uiucdcsb> <661@maize.engin.umich.edu>
Reply-To: kludge@pyr.UUCP (Scott Dorsey)
Distribution: na
Organization: Georgia College Of Universal Knowledge
Lines: 28

>In article <161200004@uiucdcsb>, kadie@uiucdcsb.cs.uiuc.edu writes:
>> It would work like this. At the spy's home base two identical
>> (digital?) tapes are make containing random noise. Then the spy goes
>> into the field with one tape recording things
>> by XORing information onto the tape.
 
 Sperry made a machine during the cold war which had a transmitter with
a reel of tape, whose synchronous motor had a local oscillator.  Incoming voice
was mixed with the tape noise such that the S/N ratio was less than one,
and then this signal was mixed with the motor synch signal (a square wave,
more or less), and transmitted.  The receiver filtered off the synch frequency
and drove another tape drive motor with it.  Then the second tape was 
subtracted from the composite frequency.
 This didn't work very well.  Channel noise would cause the system to lose
synchronization, and the task of synching the tapes up at the beginning was
tedious.  It's still in use, though I don't know why. 



Scott Dorsey   Kaptain_Kludge
SnailMail: ICS Programming Lab, Georgia Tech, Box 36681, Atlanta, Georgia 30332

   "To converse at the distance of the Indes by means of sympathetic
    contrivances may be as natural to future times as to us is a 
    literary correspondence."  -- Joseph Glanvill, 1661

Internet:  kludge@pyr.gatech.edu
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