Dirk Rijmenants provides some information about this machine. "The A-21 was invented by Arvid Gerhard Damm, a Swedish cryptographer. It uses a revolving drum with 26 alphabet strips that can be attached in any order. The 26 alphabets together are actually a scrambled vigenére square with alphabets in reversed order. A normal reference alphabet is placed in front of the drum.

To encipher, the drum advances one step further for each cipher text letter. The plaintext letter is read off at the reference alphabet and the cipher text from the drum alphabet that is currently displayed. However, there are two drum alphabets that are displayed. Which of the two drum alphabets is used, was determined by position of the reference alphabet (high or low). That position is controlled by the chain. If you take a good look at the chain you can see low and high links in it. A low link would set the reference bar in one position and a high link would set the reference bar in the other position.

The machine ‘key’ was set by changing the positions of the 26 strips on the drums and the sequence of the low and high links in the chain. If reciprocal alphabets were used, the enciphering/deciphering was even easier because it was possible to always read-off from reference to drum for both operations.

An ‘office model’ was also developed, using the same principle but with a keyboard. The name of that model is unknown to me. The firm A. B Cryptograph (predecessor of Hagelin Cryptos and Crypto AG) was actually founded to commercialize the cipher machines of Arvid Damm".

All photos by John Alexander

Contributors and Credits:

1) John Alexander <jalex_uk(at)ntlworld.com>
2) Dirk Rijmenants <dr.defcom(at)telenet.be>

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Nov 24/08