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

[Phil Karn <karn@qualcomm.com>]

>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?

This operation is ideal for computing the "hamming distance" between
two binary words, an important operation in the encoding and decoding
of forward error correcting codes. It's also used when correlating
binary streams, eg, searching for frame synchronization vectors or
despreading spread spectrum signals. All these operations are
fundamental to modern digital radio communications.

I've written software that implements a correlator, a convolutional
coder and a sequential decoder. All three make heavy use of this
operation, so I know first hand how useful it would be to have such an
instruction. The best I can do on the 386/486 when is to add the
results of table lookups on manageable pieces of the word (e.g., 8
bits at a time).

People keep assuming that NSA spends most (or even all) of its CPU
cycles on cryptanalysis. They forget that before you can attack a
cipher, you need some ciphertext. Usually this comes by radio. This
means analyzing, demodulating and decoding (as opposed to deciphering)
the digital RF modulation being used by your target. A Cray with a
library of signal analysis and demodulation programs would be ideal
for this purpose.

I would make an educated guess that this, and not cryptanalysis, is
NSA's biggest use for their Crays.  A Cray is not especially
cost-effective for cryptanalysis, at least compared with special
purpose hardware that could, say, attack DES far more cheaply.

And then there's this friend of mine who works for IDA/CRD, the NSA
think-tank in Princeton. His specialty is digital signal processing,
often using Crays. As a lark, he once demodulated some amateur packet
radio signals that were used in "Star Trek IV" as background sound
effects. Great fun.  Another time he helped the Russians demodulate
some telemetry signals from their "Vega" Venus balloon probe. Sucked
the bits right out of the noise.

Phil