Discriminant Book

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The Discriminant Book (German: Kenngruppenbuch; literally: Groups to identify the key to the receiver) shortened to K-Book (K. Buch), and also known as the indicator group book or identification group book was a secret distribution list in booklet form, which listed trigraphs in random order. The Kenngruppenbuch was introduced in May 1937, and used by the Kriegsmarine (German War Navy) during World War II as part of the Naval Enigma message encipherment procedure, to ensure secret and confidential communication between Karl Dönitz, Commander of Submarines (BdU) in the Atlantic and in the Mediterranean operating German submarines. The Kenngruppenbuch was used in the generation of the Enigma message key that was transmitted within the message Indicator. The Kenngruppenbuch was used from 5 October 1941, for the Enigma Model M3, and from 1 February 1942 exclusively for the Enigma M4. It must not be confused with the Kenngruppenheft which was used with the Short Signal Book (German: Kurzsignalbuch).