shift registers??

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2 total messages Started by Hal Feinstein Fri, 14 Aug 1992 13:59

#705

Author: Hal Feinstein
Date: Fri, 14 Aug 1992 13:59

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If you havn't seen it, there is an excellent article on what the
Canadian codebreakers are up written up in Cryptologia. Shows some
of their organization, their Cray XMP and other things. What I found
startling was the time they spend training their people on advanced shift 
register theory!  Are these devices still that popular? I would have expected 
them to have been replaced with computer type algorithms certainly within 
the last decade?  Indeed why is their so much interest in them and not in 
things like PKS?

Re: shift registers??

#773

Author: Simon Read
Date: Mon, 17 Aug 1992 11:22

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In article <1992Aug14.135934.27422@linus.mitre.org>, hal@gateway.mitre.org
writes:

> Are these devices still that popular? I would have expected 
> them to have been replaced with computer type algorithms certainly within 
> the last decade?  Indeed why is their so much interest in them and not in 
> things like PKS?

Shift registers are still used very extensively in encrypting communications
links. The reason for this is that it is near impossible to achieve the data
rates (throughput) required using software. A small shift register and some logic
gates can be designed to achieve acceptable cover times if the keys can be kept
secret.

That's where PKS comes in, that allows key exchange over insecure channels (so
long as you've set it up right). The speed of this is not terribly important, so
they can be coded in software (though low-cost hardware is available).

In addition "goverment users" tend to be a bit suspicious of these mathematical
PKS techiniques and seem to prefer old-fashioned solutions.

As an illustration the fastest modular-exponentiator chips (which are quite
expensive at about $1000 a throw) can achieve about 64kbps, rising to 128kbps
soon. (modular-exponentiation is the fundamental step in RSA,
Diffie-Hellman, El Gamal etc). Shift register chips which are much smaller
costing $100 in small quantities, can achieve throughput in the order of Mbps.

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