Discriminant Book

[1] 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.

[2] After 1 May 1937, the Kriegsmarine had stopped using an Indicating system with a repetition of message key within the indicator, a serious security flaw, which was still being used by the Luftwaffe (German Airforce) and Heer (German Army) at the beginning of 1940, making the Naval Enigma more secure.

On 9 May 1941, when a version of the K Book was recovered from U-boat U-110, Joan Clarke, and her compatriots at Hut 8, the section at Bletchley Park tasked with solving German naval (Kriegsmarine) Enigma messages, noticed that German telegraphists were not acting in a random way, which they were supposed to when making up the message Indicator.

[2] The second half consists of the group list (German:Gruppenlist) where the trigrams are sorted in alphabetical order.

By means of the Assignation list (German:Zuteilungsliste) told the radio man which table he should use for a particular cipher net.

A properly self-reciprocal bipartite digraphic encryption algorithm was used for the super-encipherment of the indicators (German:Spruchschlüssel) with basic wheel settings[4] The Enigma Cipher Keys called Heimische Gewässer (English Codename: Dolphin), (Plaice), Triton (Shark), Niobe (Narwhal) and Sucker all used the Kenngruppenbuch and bigram tables to build up the Indicator.

[4] The first trigraph was taken from the Key Identification Group table (German:Schlüsselkenngruppe), from the Kenngruppenbuch as determined in the Zuteilungsliste.