Two encrypted letters written by Spanish king Ferdinand II of Aragon baffled historians. Thanks to secret service codebreakers, the mystery is now solved.

Although I have occupied myself with unsolved cryptograms for at least a decade, this mystery has never come to my attention: two encrypted letters from Spain written in the early 15th century.

Apparently, these two letters made it to the Spanish press in 2015, when they were on display in the Army Museum of Toledo near Madrid. Here’s an article about the letters (in Spanish).

The two letters were sent by no less than Spanish king …


… Ferdinand II of Aragon (1452-1516) to his general …


… Gonzalo de Córdoba (1453-1515) in 1502 and 1506. Here’s a scan of one of the letters:


To my regret, the rest of this message and the other letter are not available.

After the letters became publicly known in Spain, there were attempts to break the encryption, but to no avail. Finally, yesterday the Spanish newspaper ABC reported that the two Ferdinand letters have been solved (thanks to blog reader Carmen for the hint). Apparently, codebreaking specialists of the Spanish secret service CNI (Centro Nacional de Inteligencia) managed to solve the mystery. According to ABC, this is the key they found:


Apparently, this is a homophonic cipher (i.e., for some letters there several ciphertext symbols to choose from). It is not a nomenclator, because none of the ciphertext symbols stands for a whole word. Homophonic ciphers are hard to break if used properly. The most famous homophonic cipher ever broken is the first encrypted message of the Zodiac Killer.

Unfortunately, I have no information about the method the Spanish secret service codebreakers used for their successful decipherment. Perhaps, they started by guessing words, as is indicated in the following diagrams that are depicted in the ABC article:


When googling for more information about this story, I found a recent article published by the Spanish newspaper El Confidencial. According to this article, the letters of Ferdinand II of Aragon had been broken before by German-Spanish scientist Gustave Bergenroth in 1862. This is far from surprising, as Bergenroth is known as a pioneer of historical codebreaking (details are available in my book Codeknacker gegen Codemacher). He deciphered hundreds of messages he found in Spanish archives. Here’s his solution of the two Ferdinand cryptograms:


Did the CNI codebreakers know this document? If so, they didn’t break the encryption but simply used a key that was already available. Maybe, a reader can tell us more about this.

Further reading: Top 50 crypto mystery solved: Thomas Ernst deciphers Fredinand III’s encrypted letters


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Kommentare (7)

  1. #1 Klaus Schmeh
    3. Februar 2018

    The BBC has published an article about the Ferdinand letters, too:

  2. #2 S.Tomokiyo
    3. Februar 2018

    I also reported it in my article.
    CNI used some deciphering on the manuscript.

  3. #3 Thomas
    4. Februar 2018

    Arturo Quirantes published his article “Descifrando nuestra historia” already in 2010: .
    Here he mentioned the “clave del gran capitan”, which is the sheet containing the key he found in a library in Spain (see the image above), and Bergenroth. There is no doubt that the Spanish secret service is capable to find such an online published article and the mentioned sheet…

  4. #4 Jerry McCarthy
    England, Europa.
    4. Februar 2018


  5. #5 Thomas
    4. Februar 2018

    The key as shown above is provided online:

  6. #7 Carmen
    3. April 2018

    Yes, the CNI codebreakers did know about Bergenroth decipherments. But what the CNI members decoded was another part of hundred of codes. The Hispanist German Bergenroth only decoded a part of some ciphers. And the Spanish Archives made him promise to leave a copy of his decipherments. Some of these decodings are also found in the British Archives under the epigraph “Transcripts from Spanish Archives, Series I 1485-1555″ (“Bundle 3 and Key to ciphers 1527-1534″.)
    Bergenroth was living in UK and left some of his transcripts behind.
    You can find more information here (in Spanish)
    Kind regards