Double letter swap board

The double letter exchange table (full title: double letter exchange table for identification groups ) was a secret code table at the time that was used in the secret communications of the German navy during the Second World War . This involved a total of ten different sets of involutive bigram tables, which indicated corresponding encodings for all 26² = 676 possible double letters from AA to ZZ. The exchange table set to be used was described by individual passwords such as source , stream , river (authentic spelling at the time), stream , bank , estuary , sea or pond . Each set consisted of nine different exchange tables , designated as table A , table B , and so on up to table J (without table I ).

Spell key agreement

In contrast to the ENIGMA I key machine used by the army and air force , the German Navy used a cryptographically stronger model, which it called "Key M". These were the ENIGMA M3 and, from February 1, 1942, the ENIGMA M4 , which was used exclusively for radio communications between German submarines and its commander (BdU). In addition to the use of a higher number of rollers used in the roller set, the Kriegsmarine achieved increased cryptographic security primarily through the method used for key agreement .

Exchange boards

The swap board plan prescribed the swap board to be changed daily.

As already mentioned, the Navy differentiated between different sets of exchange boards, which were identified by individual passwords (as stated above), for example "River" (authentic spelling at the time). Over time, older sentences were replaced by new ones. Each exchange table set contained nine different exchange tables, labeled from A to J (without I). Each individual swap table represented corresponding encodings on the front and back of a sheet in tabular form for all 26² = 676 possible double letters from AA to ZZ, for example AA-PQ, AB-CH, AC-OS and so on to ZZ-NG. So it was a bigram substitution . The swap tables were designed so that the swaps were involutorial , that is, if, for example, the double letter (bigram) AA was swapped for PQ, then PQ was swapped for AA. This made it easier to use the boards, because you didn't have to differentiate between encryption and decryption . At the same time, however, this property also represented a weakness of the system, as it made cryptanalysis easier.

Cryptanalysis

U 110 and HMS Bulldog on May 9, 1941

The British code breakers in English Bletchley Park managed - apart from isolated successes in 1940 - the slump in the key M until May 1941 after hijacking of the German submarine U 110 together with an intact M3 machine and all the secret documents, including the exchange boards through the British destroyer HMS Bulldog on May 9, 1941.

literature

• Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .
• Ralph Erskine : Captured Kriegsmarine Enigma Documents at Bletchley Park . Cryptologia . Rose-Hulman Institute of Technology. Taylor & Francis, Philadelphia PA 32.2008,3, pp. 199-219, ISSN  0161-1194 .
• Ralph Erskine: The German Naval Grid in World War II . Cryptologia, pp. 16: 1, 39-51, 1992. doi: 10.1080 / 0161-119291866757
• Hugh Sebag-Montefiore : ENIGMA - The battle for the code . Cassell Military Paperbacks, London 2004, ISBN 0-304-36662-5

Web links

• Authentic set of double-letter swap boards for password "source". Retrieved June 9, 2016.
• Double letter exchange table "Password: River" (authentic spelling)
• ENIGMA procedures (English)

Individual evidence

1. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 115.
2. ^ Hugh Sebag-Montefiore: ENIGMA - The battle for the code . Cassell Military Paperbacks, London 2004, p. 225. ISBN 0-304-36662-5
3. ^ Hugh Sebag-Montefiore: ENIGMA - The battle for the code . Cassell Military Paperbacks, London 2004, pp. 149ff. ISBN 0-304-36662-5