From: jito@netcom.com (Joichi Ito)
Date: Fri, 28 Jan 94 09:52:18 PST
To: cypherpunks@toad.com (Cypherpunks List)
Subject: Re: Quantum cryptography
Message-ID: <9401281749.AA23308@iikk.inter.net>
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I'm taking the liberty of forwarding this note here because email to
jdblair@rogue.cas.muohio.EDU bounces so I couldn't thank John or ask for
permission. It was the only reply I got, but I think it is really
interesting so...

 - Joi


>From: jdblair@rogue.cas.muohio.EDU (John Blair)
>Message-Id: <9401280824.AA08537@ rogue >
>Subject: Re: Quantum cryptography
>To: jito@netcom.com (Joichi Ito)
>Date: Fri, 28 Jan 1994 03:24:41 -0500 (EST)
>In-Reply-To: <9401280501.AA02095@iikk.inter.net> from "Joichi Ito" at Jan 28,
>94 02:00:22 pm
>X-Comment: The enlightened man is one with the subject. -Mumon
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>
>> 
>> Does anyone know how quantum cryptography works?
>> 
>>  - Joi
>> 
>> 
>> --
>> true name:                 <Joichi Ito>
>> closest email address:     <jito@netcom.com>
>> closest fax number:        <+81-3-5454-7218>
>> current physical location: <Tokyo>
>> travel path:               <.>
>> mosaic home page:          http://iikk.inter.net/
>> --
>> finger jito@netcom.com for PGP Public Key, RIPEM Public Key
>> --
>> 
>
>Joi,
>
>Chances are you're recieving a lot of answers to this, but here's one anyways.
>
>This comes from:  Schneier, Bruce, _Applied Cryptography_, pp 408-410,
>John Wiley & Sons, Inc, New York.  pp 408-410
>
>Quantum cryptography relies on the property that one cannot know the
>position and the velocity of a particle at the same time to prevent
>evesdropping.  Any evesdropper will neccessarily disturb the message in a
>detectable way.  Experimental quatum cryptography relies on polarized light.
>
>If a pulse of horizontally polarized photons is sent through a
>horizontally polarized filter, 100% of the light gets through.  Pass it
>through a filter 45 degrees off, and each partical has a 50% chance of
>making it through, and no chance of making it through a filter 90 degrees off.
>
>1) Alice sends Bob a string of photon pulses.  Each of the pulses is
>randomly polarized in one of four directions: horizontal, vertical,
>left-diagonal, and right-diagonal.
>
>For example, Alice sends Bob:  ||/--\-|-/
>
>2) Bob has a polarization detector.  He can set his detector to measure
>horizontal and vertical polarization, or he can set his detector to
>measure diagonal polarization.  He can't do both; quantum mechanics won't
>let him.  Measuring one destroys any possibility of measuring the other. 
>So, he sets his detectosr at random, for example:  x++xxx+x++
>
>When Bob sets the detector correctly, he records the correct answer.  The
>rest of the time, he gets a random result.  Say he gets: ||/--\-|-/
>
>3) Bob, over an insecure channel, tells Alice what settings he used.
>
>4) Alice tells Bob which settings were correct.  In this example, 2,6,7,9.
>
>5) Alice and Bob keep only those settings that were correct.  In this
>example, they keep:  *|***\-*-*
>
>According to a pre-arranged code, such as 1 = horizontal and
>left-diagonal, and 0 = vertical and right-diagonal, they have sent the
>bits: 0011
>
>Alice will need to send 2n pulses for every bit transmitted, since Bob
>will be correct 50% of the time, on the average.  These bits can be taken
>as the key to a conventional algorithm, or enough can be sent to generate
>a one-time pad.
>
>6) Alice and Bob compare a few bits in their strings.  If there are
>discrepancies, they know they are being bugged.  If there are none, they
>discard the bits they used for comparison and use the rest.
>
>
>For a good overview (according to Schneier) see:
>
>C. H. Bennet, G. Brassard, and N. D. Mermin, "Quantum Cryptography,"
>Scientific American, v. 68, n. 5, 3 Feb 1992, pp. 557-559
>
>C. Zimmer, "Perfect Gibberish," Discover, v. 13, n. 12, Dec 1992, pp. 92-99.
>
>
>Schneier says British Telecom has used this system to send bits over a 10km
>fiber-optic link.
>
>see:
>W. Brown, "A Quantum Leap in Secret Communications," New Scientist, n.
>1585, 30 Jan 1993, p. 21.
>
>
>Hope this made sense.  Its really quite amazing to think about.
>-john.
>
>


--
true name:                 <Joichi Ito>
closest email address:     <jito@netcom.com>
closest fax number:        <+81-3-5454-7218>
current physical location: <Tokyo>
travel path:               <.>
mosaic home page:          http://iikk.inter.net/
--
finger jito@netcom.com for PGP Public Key, RIPEM Public Key
--