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From: "R. A. Hettinga" <rah@shipwright.com>
Subject: Re: Optical analog computing?
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Status: RO
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To: fork@example.com
Subject: Re: Optical analog computing?
From: Dave Long <dl@silcom.com>
Sender: fork-admin@xent.com
Date: Wed, 02 Oct 2002 11:09:34 -0700



> > "the first computer to crack enigma was optical"
>
> Computing with interference patterns, etc, makes perfect sense, but Enigma
> was cracked by building mechanical systems that were essentially Enigma
> machines and brute-forcing.

Look for "Zygalski sheets".

By Koerner's* narrative, it wound up
being a hybrid affair: Bletchley had
mock Enigmas which cycled through the
Enigma's ~18k starting positions in a
quarter of an hour, but the Germans
started using a plugboard which then
had ~1.5x10^14 possibilities.  The
Poles noticed that there were some
patterns in the messages which were
only possible via certain plugboard
settings, and so:

> When we have collected several such cards corresponding to different
> initial signals ..., we place them in a pile so that the squares
> corresponding to the same daily settings are aligned and shine a
> light beneath the pile.  Only those squares which let the light
> through will correspond to possible daily settings.

So the brute force hardware allowed
precalculation of "optical" computing
devices which then narrowed down the
possibilities enough for brute force
to again be used for daily decodes,
until:

> On 10 May, the Germans invaded France and, on the same day, in
> accordance with the best cryptographic principles, they changed their
> Enigma procedures in such a way that the 1560 Zygalski sheets, each
> with their carefully drilled 1000 or so holes, became just so much
> waste cardboard.

-Dave

::::::

> > "the first synthetic-aperture-radar processor was optical"

This is also easy to believe, given Dr.
Elachi's description of the 1981 Shuttle
Imaging Radar:

> The received signal is recorded on an optical film which is retrieved
> after landing.  The film ... is then processed in an optical correlator
> to generate the final image.

which makes sense, as one wishes to shift
each component of the return in proportion
to its frequency, for which one presumably
needs a glorified prism.

> Alternatively, the ... signal can be digitized and then recorded on
> board or transmitted to the ground via a digital data link.  This was
> the case with the ... sensor flown in 1984.

::::::

* Koerner, _The Pleasures of Counting_,
in which various aspects of the Enigma
decoding cover four chapters, of which
I quote from two sections of one:
14.2: Beautiful Polish females, and
14.3: Passing the torch
> Churchill's romantic soul loved the excitement and secrecy surrounding
> Bletchley.  He relished the way that
> > [t]he old procedures, like the setting up of agents, the suborning
> > of informants, the sending of messages written in invisible ink,
> > the masquerading, the dressing-up, the secret transmitters, and the
> > examining of the contents of waste-paper baskets, all turned out
> > to be largely cover for this other source, as one might keep some
> > old-established business in rare books going in order to be able,
> > under cover of it, to do a thriving trade in pornography and erotica
> ...
> Looking at the disparate, unkempt and definitely unmilitary crew
> formed by his top code-breakers, he is said to have added to his head
> of Intelligence "I know I told you to leave no stone unturned to find
> the necessary staff, but I did not mean you to take me so literally!"

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-- 
-----------------
R. A. Hettinga <mailto: rah@ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
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"... however it may deserve respect for its usefulness and antiquity,
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experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'


