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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. -----BEGIN PGP SIGNATURE----- Version: 2.3 iQCVAgUBLhX/qtCZASdT8NoBAQF8SAP/V5FKgEaCk1GQXV9rrK+AMry2Bzb9Xlyu bYMqjN94mAqqkNOe1r2ChmUF4kleTUMxdx1Krje3xhLDPL31HH4lvJ386sm6Ogrm /iu/TgjoSnGbMYtoq+C2ZJacA/NBDzItTeUaZgkWRS62Emo/cFIGarT130clL8/x HnNbtdGtSOE= =VVZZ -----END PGP SIGNATURE-----