From: Ian Farquhar <ifarqhar@laurel.ocs.mq.edu.au>
To: cypherpunks@toad.com
Message Hash: 4e7733c9f10c198aa9f641e47cefe69c60bb719d46a19aaf7ce248570f71e833
Message ID: <199407030001.AA14425@laurel.ocs.mq.edu.au>
Reply To: <m0qJvy4-0002FgC@chinet>
UTC Datetime: 1994-07-03 00:01:34 UTC
Raw Date: Sat, 2 Jul 94 17:01:34 PDT
From: Ian Farquhar <ifarqhar@laurel.ocs.mq.edu.au>
Date: Sat, 2 Jul 94 17:01:34 PDT
To: cypherpunks@toad.com
Subject: Re: Dr. Dobbs Dev. Update 1/5 July 94 & Schneier
In-Reply-To: <m0qJvy4-0002FgC@chinet>
Message-ID: <199407030001.AA14425@laurel.ocs.mq.edu.au>
MIME-Version: 1.0
Content-Type: text/plain
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> Feedback with Carry Shift Registers (FCSRs): Linear
>Feedback Shift Registers (LFSRs) have been the workhorse of
>military cryptography for years. Goresky and Klapper have
An interesting thought hit me when reading this. The "classic"
Cray series (Cray-1, X-MP, Y-MP) all have a rather curious instruction
generally known as population count. All it does is to take a register
and count the number of one bits in it, and return that count. Originally
I could never figure out a use for this, but later was told that it was the
"canonical NSA instruction", and was consistently demanded by almost all
military SIGINT operations.
On reading this, I realised that one possible use was to implement a
vectorized version of a LFSR. Take a vector register (the shift register),
AND it with a mask of the taps into another vector register, and then
do a population count to determine the carry in.
Just a thought. It's the only plausable use that I have yet thought of
for this instruction. Has anyone else got any ideas?
As for military ciphers having been "the workhorse of military
cryptography for years", I am reminded (with some amusement) of the
structure of A5. I wonder if all of the fuss about secrecy was not
about the almost non-existant security of the cipher, but simply it's
similarity to more sophisticated military ciphers?
Ian.
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