Big cipher

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One page of a nomenclator (from 1690) similar to the Great Cipher

The Great Cipher (French: Grand Chiffre ; English: Great Cipher ) was a special nomenclator , which in the 17th century by the French cryptologist Antoine Rossignol (1600–1682) and his son Bonaventure Rossignol in the service of the French King Louis XIV. (1638 –1715) in whose Cabinet noir (German: Black Chamber) was developed. It was so effective that it was in use for almost 200 years.

A special feature of the method and special protection against unauthorized deciphering this Nomenklators is that he, in addition to the usual code groups still Blender (fill characters without meaning), homophones for syllables ( replacement of plain text syllables by several different ciphertext characters) and traps (control characters which, for example, delete the previous code group). During this time, messages were normally encrypted by simply replacing the individual letters with numbers or symbols. In the simple form (word / number) this encryption system was very easy to crack. However, Rossignol developed an encryption system in which syllables and whole words were also replaced by numbers. He complicated the system even more by assigning the numbers randomly instead of a numerical system with an ascending sequence of numbers. In the case of intercepted letters, people tried to guess what the numbers should mean, but assumed a wrong approach, since in reality one should have looked for phrases. Rossignol also inserted nonsensical sentences with misleading content into his encrypted messages. To mark this there was a - also coded - character that was supposed to announce the following nonsense to the recipient. The decipherment was only possible with code tables on which the - alphabetically ordered - word and the corresponding numerical value were recorded. These were subject to the utmost secrecy. Any unauthorized person caught in France with a copy or using the code was executed for it.

After the death of the king and Antoine's grandson, Antoine-Bonaventure Rossignol , the Great Cipher was forgotten and the documents encrypted with it could no longer be read. It was not until 1890 that the French cryptanalyst Étienne Bazeries succeeded in deciphering the Great Cipher . This enabled a number of secret documents from the time of Louis XIV to be read again, which gave historians interesting insights into the conditions at that time. Among other things, a letter was found encrypted with the large cipher , the plain text of which was determined by Bazeries as possible evidence that the man with the iron mask could be Vivien de Bulonde .

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