Shipyard key

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The shipyard Key was one of the German Navy employed hand key method . It was used by smaller naval units such as port ships that did not have a machine key ( Enigma-M3 or Enigma-M4 ), but also by large warships in case they had to communicate with the small ones.

Procedure

The yard key was basically a polyalphabetic bigraphic bipartite substitution, combined with a simple seriation of the plaintext . This was written in groups of five, which were arranged one below the other. So you got the plain text in five columns and many lines. The letters directly below each other in two adjacent lines were interpreted as bigrams and replaced (replaced) by secret bigrams with the help of double-letter exchange tables ( bigam substitution tables similar to double-letter exchange tables for identification groups ). A different exchange table was used for each of the five columns. For this purpose, a (then) secret key book was issued every two months (later monthly) , which contained twenty (later thirty) double-letter exchange tables to choose from.

Each of these exchange tables represented a square matrix of 26 × 26 letters in which a secret bigram was specified for each of the 26² = 676 possible plain text charts from AA to ZZ. The assignment was chosen involutorily , so if, for example, AX was encrypted as PY, then PY was encrypted as AX. This made it easier to work with the exchange tables, because you didn't have to differentiate between encryption and decryption , but at the same time this measure weakened the cryptographic security of the process. In addition, one of the two plain letters never reappeared anywhere in the secret bigram, i.e. AX was never encoded as a bigram in which an A or an X appeared. This possibly helped with the practical application of the shipyard key, because it reduced mix-ups, but represented a further cryptographic weakening.

As a result of the bigram substitutions, groups of five were again obtained as ciphertext, which were usually transmitted by radio in Morse code . The authorized recipient of the secret message could decrypt it and read the original plain text using the key known to him and by reversing the procedural steps.

Cryptanalysis

As early as April 1940, the British interception stations registered radio messages for the first time, which - as it turned out a few weeks later - were encrypted using the shipyard key. They were broken regularly and without much delay by Codebreakers in Bletchley Park (BP) . In total, between March 1941 and February 1945, around 33,000 German radio messages encrypted with the shipyard key were intercepted and deciphered in BP .

Not only the intelligence content of the deciphered texts was interesting for the British, but above all the possibility to use the plain texts as crib to decipher Enigma radio messages .

literature

Web links

Individual evidence

  1. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 400.
  2. ^ Hugh Sebag-Montefiore: Enigma - The battle for the code . Cassell Military Paperbacks, London 2004, p. 214. ISBN 0-304-36662-5 .
  3. ^ Gordon Welchman: The Hut Six Story - Breaking the Enigma Codes . Allen Lane, London 1982; Cleobury Mortimer M&M, Baldwin Shropshire 2000, p. 11. ISBN 0-947712-34-8
  4. ↑ Christopher Morris: Navy Ultra's Poor Relations in Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, p. 233. ISBN 0-19-280132-5 .
  5. ↑ Christopher Morris: Navy Ultra's Poor Relations in Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993, p. 235. ISBN 0-19-280132-5 .