ABC cipher

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The ABC cipher was a hand-key method used by the Imperial Army during World War I to transmit classified military messages using wireless telegraphy . The encryption took place in two stages and consisted of a Vigenère cipher with the fixed key ABC followed by a simple column transposition with a changing password . A little later the procedure for the ABCD cipher was extended, a modification with hardly noticeable effect.

history

The ABC cipher was introduced on the Western Front in the first year of the war on November 18, 1914. It was triggered by newspaper articles, for example in Le Matin , in which it was reported that the French had broken the previous German procedure and that they were able to "read" the secret German news.

The Germans had been using the old 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, which could easily be intercepted on the French side, also served as training radio messages there. The French named the German cipher after the five letters as le ciffrement UBCHI . They had made good use of the time before the war as a practice and at the beginning of the war they were very familiar 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 had learned about newspaper reports that her twin cube method by the French cracked was, it was decided hastily to a radical change in the method, replacing the old process by the ABC cipher until May 1915, ie about half a year , has been used.

Procedure

The ABC encryption process consists of two stages, a substitution (replacement of characters by others) followed by a transposition (exchange of the arrangement of the characters). For the first stage, the plain text is written in groups of three letters and the first letter of each group of three remains unchanged. All letters in the middle are replaced by the following letter in the alphabet. A becomes B, B becomes C, and so on, and Z becomes A. This corresponds to a Caesar shift by one. The last letters of each group of three are replaced by the next but one letter in the alphabet. A becomes C, B becomes D, and so on. Y becomes A and Z becomes B. This corresponds to a Caesar shift of two. In this way, an intermediate text is obtained after the first level of encryption. The same result is obtained if the plain text is encrypted using the Vigenère method with the password ABC. The advantage of this very simple cryptographic method is that it could be carried out without any tools in the head. The disadvantage is the low level of cryptographic security it offers.

In the second stage, the order of the letters is changed. In contrast to the double column transposition previously used in the double cube, one was satisfied here with a single column transposition. The intermediate text was written line by line in a rectangular matrix , the width of which was determined by the length of a password. The intermediate text was entered line by line in this matrix and then read out again column by column. The columns were not regularly taken one after the other from left to right, but rather read out more or less irregularly, controlled by the alphabetical order of the letters of the password. The result was the ciphertext , which was then transmitted by radio in Morse code .

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 had 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. Now the Caesar shift only had to be reversed. The subtext was written in groups of three. Subsequently, all letters in the middle of each group were replaced by the letters immediately in front of it in the alphabet and all letters in the back of each group were substituted by those two places in front of it in the alphabet. So you got the original plain text back.

Decipherment

The cryptanalysis and regular deciphering of the new German cipher caused little trouble for the French cryptanalyst Georges Painvin , which he succeeded in from December 1914. According to his own statement, he found the ABC cipher to be much easier to crack than the old double cube. Naturally, a single column transposition is significantly less secure and weaker than a double column transposition. And the Vigenère cipher with the fixed and extremely weak password ABC hardly offers any additional protection against decipherment and can rightly be described as a complication illusoire .

literature

Web links

  • Le chiffre ABC explanations as well as an 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. 34.
  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. ↑ Friedrich L. Bauer: Deciphered secrets - methods and maxims of cryptology . Springer, Berlin 2000 (3rd edition), ISBN 3-540-67931-6 , p. 446.