ÜBCHI

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ÜBCHI (also referred to as UBCHI by the French ) was a hand key method used by the Imperial Army at the beginning of World War I to transmit classified military messages using wireless telegraphy . The procedure is based on a double column transposition using a single password .

history

The Germans had been using this encryption method since 1912 for exercise, for example in maneuvers , and continued to use it during the first months of the war. It was a double column transposition with a single password, also known as a "double cube" . In the pre-war period, the five letters ÜBCHI, an abbreviation for “exercise cipher”, were placed in front of the encrypted exercise radio messages. The radio messages that were intercepted on the French side were also used there as training radio messages. The French named the German cipher after the five letters as le ciffrement UBCHI . The French cryptanalysts used the time before the war for practice and at the beginning of the world war they were well acquainted with the German procedure, the customs of radio operators and the military terminology and were thus able to decipher the German radio messages . After the German General Staff on newspaper reports such as the Le Matin had learned that her twin cube method by the enemy cracked was, it was decided hastily to a radical change in the method, replacing the old process on 18 November 1914 by the new ABC cipher which was used until May 1915, i.e. for about half a year.

Procedure

The encryption method is based on the well-known double cube and only used a single password for both levels of transposition. The column transposition method generally uses a rectangular arrangement, also known as a matrix , consisting of several rows (as many as are necessary to enter the plain text ) and a number of columns specified by the key word . The number of columns corresponds to the number of letters in the keyword. The plain text is entered into the matrix line by line. As an intermediate text, the individual letters are read from the matrix column by column, the order in which the columns are read out is determined by the alphabetical order of the letters in the keyword. In the second process step, the intermediate text is again entered into the matrix line by line and read out again column by column according to the sequence of letters in the password. This results in a further “scrambling” of the text. The result is the ciphertext , which was then transmitted by radio in Morse code .

It would have been much more secure, but also a little more complicated to use, if not just a single password for both levels of transposition, but to use two different passwords, i.e. a separate password with a different length for each of the two levels of transposition . The passwords should also have been changed at least every day; it is even better to use individual ones for each individual radio message. In fact, the Germans used their passwords consistently for a full eight to ten days on the entire Western Front - a mistake that made ÜBCHI easy prey for the French code breakers .

Decryption

On the receiving side, the recorded Morse code was entered as letters column by column in a rectangle of known width. Since both the recipient and the sender were in possession of the password that served as the secret key, they knew the required width of the rectangle from the length of the password. The columns were not filled from left to right, but in the order specified by the alphabetical order of the letters of the password, whereby the length of the columns, i.e. the height of the rectangle, is achieved by dividing the length of the radio message by the length of the password and rounding up if necessary was determined to the nearest natural number. The ciphertext entered column by column was then read out line by line and the original text was returned. As described above for the ciphertext, the intermediate text was entered into the matrix column by column and read out again line by line. The original encryption was thus reversed and the plaintext was available.

Decipherment

The French cryptanalysts around Georges Painvin succeeded in cryptanalysis and the frequent deciphering of the procedure, mainly due to the German mistake of using only a single password and using this uniformly on the entire western front for many days. The French had many dozen, if not hundreds, of German radio messages encrypted with the same key. Naturally, some of these were of identical length. This allowed them to break in and, from October 1914, the solution to the procedure, which in principle was not easy to crack. To do this, they also used the cryptanalytic method of multiple anagrams. Another mistake made by the Germans was their preference for pathetic passwords such as “KampfundSieg” or “MagdeburganderElbe”, which the French could easily guess.

literature

Web links

  • Le cipher UBCHI explanations as well as encryption and decryption tool (French). Retrieved June 7, 2016.

Individual evidence

  1. ↑ Friedrich L. Bauer: Deciphered secrets - methods and maxims of cryptology . Springer, Berlin 2000 (3rd edition), ISBN 3-540-67931-6 , p. 28.
  2. ↑ Klaus Schmeh: Code breakers versus code makers - The fascinating history of encryption . W3L-Verlag, Dortmund 2014 (3rd edition), ISBN 978-3-86834-044-0 , p. 33.
  3. ↑ Michael van der Meulen: The Road to German Diplomatic Ciphers - 1919 to 1945 . Cryptologia, 22: 2, 1998, p. 143, doi: 10.1080 / 0161-119891886858
  4. ↑ Klaus Schmeh: Code breakers versus code makers - The fascinating history of encryption . W3L-Verlag, Dortmund 2014 (3rd edition), ISBN 978-3-86834-044-0 , p. 34.
  5. ↑ Friedrich L. Bauer: Deciphered secrets - methods and maxims of cryptology . Springer, Berlin 2000 (3rd edition), ISBN 3-540-67931-6 , p. 446.