Card catalog (cryptology)

The card catalog, or "catalog of characteristics," in cryptography, was a system designed by Polish Cipher Bureau mathematician-cryptologist Marian Rejewski, and first completed about 1935 or 1936, to facilitate decrypting German Enigma ciphers.

Preparation of the card catalog, using the cyclometer that Rejewski had invented about 1934 or 1935, was a laborious task that took over a year's time. But once the catalog was complete, obtaining Enigma daily keys was a matter of some fifteen minutes.

When the Germans changed the Enigma machine's "reflector," or "reversing drum," on November 1, 1937, the Cipher Bureau was forced to start anew with a new card catalog, "a task," writes Rejewski, "which consumed, on account of our greater experience, probably somewhat less than a year's time." On September 15, 1938, the Germans changed entirely the procedure for enciphering message keys, and as a result the card-catalog method became completely useless. This spurred the invention of Rejewski's cryptologic bomb and Henryk Zygalski's "perforated sheets."

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