Dr. Dobbs Dev. Update 1/5 July 94 & Schneier

[Ian Farquhar]

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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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