A secret code called The Great Captain Code used by the Spanish king Ferdinand II of Aragon 500 years ago was described by Spanish national daily newspaper ABC as “one of the great mysteries in the history of Spain”. The coding system contained over 200 special characters and was used by the king to communicate with his military commander, Gonzalo Fernández de Córdoba. For centuries, no one had been able to decipher the letters, as the substitution table the king used to create the code had not been preserved.
However, after historians were unable to crack the codes, the Museum turned to the CNI. A small piece of text deciphered by de Córdoba at the foot of one of the documents provided a vital clue that helped the CNI team unlock the code. The CNI, in fact, has not discovered the hidden code for 500 years, because it had already been partially analyzed and deciphered before. The copy is kept by the BNE at the Recoletos headquarters under the title 'Cartas de Gonzalo Fernández de Córdoba to different people [Manuscript]'. Both are the same code although they are not identical, because the first one is a fragment: the original document with the integral key has not yet appeared.
In total, 88 different symbols and 237 “combined letters” were used to create the ”Great Captain code”. The code is a precursor of the so-called “Vigenère cipher”, but an advanced form that was not widely used until the 17th century. According to wikipedia, the Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers based on the letters of a keyword. It is a form of polyalphabetic substitution. Though the cipher is easy to understand and implement, for three centuries it resisted all attempts to break it; this earned it as 'the indecipherable cipher'. Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. Friedrich Kasiski was the first to publish a general method of deciphering a Vigenère cipher in 1863.
The decoded letters dated from the time of the second French invasion of Naples, which the Spanish controlled at the time. They therefore represent a key moment in Spanish history, when Spain and France were fighting for dominance in the Mediterranean region.
The Museum noted the “meticulous and detailed” instructions given to de Córdoba by King Ferdinand: In addition to directions on troop movements, the King also discusses the administration of justice and even promotes marriage between the local widows and the Spanish military in an attempt to achieve greater social integration. Ferdinand also criticizes the commander for launching diplomatic initiatives without consulting him, according to the BBC.
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