This cipher machine was a simple one of purely mechanical construction, but it produced a cipher with a long period.
A drum, containing 29 mixed cipher alphabets, advanced two positions each time the machine was used to encipher a letter.
An alphabet in normal order contained the plaintext letters. A window high enough to expose two of the alphabets on the drum was present on the device. The plaintext alphabet was moved in an irregular sequence to cover either the top or the bottom alphabet of the two on the drum that were behind the window, but whether the normal alphabet was the high one or the low one, it was always the one used for plaintext.
Thus, the movement of the bar with the plaintext alphabet simply controlled which of two of the 29 cipher alphabets would be used at each step.
The irregular movement of this bar was controlled in two steps. First, a 25-pin pinwheel advanced with each letter enciphered. Active pins on this pinwheel advanced a chain with a variable number of links; some of those links were shaped to call for the plaintext alphabet bar to be in its lower position, some were shaped to call for it to be in its higher position.
Unfortunately, the cipher produced by this machine as described so far is quite weak; if one knows that an A-22 has been used, one simply divides the encrypted message into groups of 29 letters. Then, for each column of letters at that spacing, only two alphabets are used, and this fact diminishes the benefit of the elaborate mechanism devoted to alternating between those two. Instructions provided with the machine, however, suggested advancing it two or three steps after enciphering some letters; those who went to this extra effort would have obtained a greater degree of security.
Table of Contents