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An American Patent

U. S. Patent 4157453 is one of the few patents that were secretly issued to people working for the NSA or one of its predecessor agencies that has now been publicly issued in normal fashion, many years later.

It was filed on October 10th, 1944, but issued on June 5th, 1979. The inventor was Leo Rosen, known to history as the one who had the inspiration to realize that the Japanese cipher machine codenamed PURPLE used telephone stepping switches.

This patent refers only to scrambling the order of bits in a teletypewriter signal, without inverting any bits. However, in addition to shuffling the bits within a single character, as the T-52 did, this device moved bits between characters.

Originally, I was only able to see the claims of this patent, but the text of it has since become available. It turns out that it referred to buffering two or four 5-level-code characters, recieved serially, in banks of relays, transposing the stored bits, and then transmitting the result. Given the technology of the time, commutator switches were used to access relays serially; today, a designer thinking of how to perform such an operation would no doubt think of shift registers.

Having just seen a brief description of the purpose of the invention, I had supposed that this patent could have been for a system that was actually applied to a combined time and frequency division voice scrambler, with its application disguised for security.

However, as a telecipher device, one of the simplest ways in which to accomplish this kind of scrambling would be to do the following: use pinwheels to scramble the bits within a character; then, subject these bits to a fixed pattern of delays, so that they are distributed among two to five different characters; then, perform a second pinwheel-controlled scramble of the bits within each character.

To decipher, one first inverts the second scramble. Then, one delays the bits in a complementary fashion to that used for enciphering: if bits 1 to 5 are subject to delays 0 1 0 1 0 on enciphering, then use delays 1 0 1 0 1 on deciphering, for a net delay of every bit by 1 character; if bits 1 to 5 are subject to delays 0 1 2 3 4 on enciphering, use delays 4 3 2 1 0 for deciphering, for a net delay of 4 characters. Then, invert the first scramble, with the pinwheels controlling it offset so that their scramble is delayed to match the net delay of the enciphering delay stage and its deciphering complement.

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