The card catalog, or "catalog of characteristics," in cryptography, was a system designed, and first completed about 1935, by Polish Cipher Bureau mathematician-cryptologist Marian Rejewski to facilitate decrypting German Enigma messages. The card catalog was produced over more than a year's time, using the cyclometer invented by Rejewski about 1934.

When the Germans changed the Enigma machine's "reflector," or "reversing drum," on November 1, 1937, the Cipher Bureau was forced to start over again and produce 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."

References

• Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984, especially pp. 242 and 284-87.