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The Swedish SA-1

This telecipher device combines some of the features of the SZ-40 and T-52, in a design that is secure yet economical.

The device has two sets of five pinwheels, of lengths 19, 21, 23, 25, and 26. ( Toby's Cryptopage, now available again after a hiatus, also discusses the TA-1, a later version with larger pinwheels of lengths 26, 29, 31, 33, and 35.) The pins are pushed in or pulled out by the user, and the five bits of each plaintext character are modified by being subjected to an exclusive-OR operation with the current pin of the corresponding pinwheel from each of the two sets.

The first set of pinwheels steps one position for every character enciphered. An additional position on the pinwheels, besides the one used to XOR the plaintext, is sensed (two positions earlier). That alternate output, from all five pinwheels, is XORed together, creating a single bit; if 0, the second set of pinwheels steps only once; if 1, they step twice.

The second set of pinwheels is also sensed at the same alternate position, and the output from all of them is combined by XOR. This time, the result, if 1, causes the entire output ciphertext character to be inverted.

The following diagram:

illustrates how it works.

This design allows two identical modules to be used in the device, thus making manufacture more economical. It achieves irregular stepping, but only ten, not twelve, pinwheels are required.

The source for the information on this page is the home page of Torbjörn Andersson, entitled Toby's Cryptopage, which has an entry in my Links section. It is well worth a visit, and includes original photos of many of the machines discussed here; not just the SA-1, but also the T-52 and the B-211.

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