Choice word

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As a choice word ( English optional word ; also: extension word or filler ) is in cryptology , especially in the context of the Second World War by the German Wehrmacht on the encryption used their message traffic rotor machine Enigma , a possible random to be selected word refers to the beginning or the end of the plain text of a radio message is inserted. This should make the opposing cryptanalysis more difficult and theDecipherment of the ciphertext can be prevented.

application

According to the then secret service regulation The Key M - Procedure M General , this measure primarily served to bring radio messages “to different lengths” in this way ( the Enigma-M4 was meant by key M ). Often plain text with the same content had to be sent to different recipients in encrypted form. For this purpose, they were encrypted with different keys according to the various key networks, which results in different ciphertexts with the same text length. If the opponent now sees several different, but equally long, ciphertexts almost simultaneously, possibly from the same sender, he may suspect a “ ciphertext-ciphertext compromise ”. The British Codebreakers in Bletchley Park (BP), England, knew and valued this case and called it a kiss ( English for "kiss" ). Such a kiss was considered the ideal way to break the radio message, even better than a “ crib ”, ie a presumed text passage.

By inserting optional words of different lengths at the beginning or end of the saying, occasionally also at the beginning and at the end, the sayings are brought to different lengths and the opponent is thus denied this opportunity or at least made more difficult. The length of an optional word was usually between four and fourteen letters. The German regulations cite “water buckets, telephones, oak trees, roof ridge, wardrobe” as examples. In practice there were occasional optional words of length three such as ABC or XXX, but also very long monstrous words such as DONAUDAMPFSQIFFAHRTSGESELLSQAFTSKAPITAEN or HOTTENTOT [T] EN POTENTATEN TANTEN ATTENTAETER .

The reason to choose from a set words more or less meaningful words of the German language, often composites , ie composite nouns , and not any meaningless random text as CIHJT UUHML is to see that the authorized recipient of the message could thus be signaled that he had decrypted the encrypted radio message without errors . However, it was stipulated that an optional word could not be in connection with the actual message content of the radio message. And of course it was not allowed to “violate discipline and order” or contain “personal communications”.

Optional words were also used in connection with other cipher machines, for example with the "secret writer" Siemens & Halske T52 .

effect

Indeed, the introduction of electoral words, first observed by the British in North Africa in December 1942 , made the codebreakers ' work difficult . However, by this time they had already penetrated the German methods for so long and so deeply and were so familiar with their habits that the further successful break could no longer be stopped in this way. It was annoying for them when they had to try out a text passage suspected at the beginning of the radio message in several positions due to the unknown length of the choice word, but the deciphering could not be prevented. The British post-war evaluation of this German measure is too little and too late ( German  “too little and too late” ). As with many other German measures to strengthen the cryptographic security of the Enigma , such as the introduction of the cryptographically strong methods of the Enigma clock or the plug-in reversing roller D , the choice word method also failed because it was not introduced abruptly and across the board and because she was late.

"Introduced in 1940 on a wholesale scale, Wahlworts might have knocked out the infant Crib Room before it had got properly on its feet."

"Introduced on a large scale [in] 1940, optional words could possibly have paralyzed the crib room, which was [then] in its infancy , before it could have really gotten to its feet."

literature

  • John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, pp. 211-216, ISBN 978-0-9557164-3-0
  • Tony Sale : The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 93, PDF; 0.4 MB , accessed August 24, 2018.

Web links

Individual evidence

  1. ^ A b John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, p. 211. ISBN 978-0-9557164-3-0 .
  2. ^ OKM: The Key M - Procedure M General . Berlin 1940, p. 10.
  3. ↑ OKM: Communication regulation of the Kriegsmarine - Annex 19 . Berlin 1943, p. 26.
  4. ↑ a b c OKM: The Key M - Procedure M General . Berlin 1940, p. 9.
  5. ^ 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 .
  6. ↑ John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, p. 213. ISBN 978-0-9557164-3-0 .
  7. ^ John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, p. 214. ISBN 978-0-9557164-3-0 .
  8. ↑ Bengt Beckman: Arne Beurling and Hitler's secret writer. Springer-Verlag 2006, ISBN 3-540-31290-0 , p. 180.
  9. ^ John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, p. 215. ISBN 978-0-9557164-3-0 .
  10. ↑ John Jackson: Solving Enigma's Secrets - The Official History of Bletchley Park's Hut 6. BookTower Publishing 2014, pp. 215-216. ISBN 978-0-9557164-3-0 .