The autokey cipher is a variant of the Vigenère cipher that was once considered unbreakable. With today’s means it can be deciphered. Can a reader solve the challenge I’m introducing today?

Most readers of this blog certainly know the Vigenère cipher, a manual encryption method invented in the 16th century. A well-known variant of the Vigenère, which incorporates the plaintext into the key, is referred to as “autokey cipher”. The following explanation is taken from Wikipedia. More details can be found on the website of the American Cryptogram Association (ACA).


The autokey cipher

The autokey cipher is based on a keyword, for example QUEENLY. To encrypt the message ATTACK AT DAWN, the keyword is written below the plaintext in the following way:

Key:        QUEENLY

Next, the first letters of the plaintext are written behind the keyword:


The ciphertext is generated by adding the plaintext to the key (A=1, B=1, C=2, …):


An autokey implementation is available on CrypTool Online.


A challenge

The autokey cipher was once called “le chiffre indéchiffrable” (“the unbreakable cipher”), but in the 19th century methods to break it were discovered. Nevertheless, the autokey cipher is considerably more secure than standard Vigenère (the Vigenère cipher once was considered unbreakable, too).

The autokey cipher is mentioned frequently in the literature, but I have never seen it being used in practice. If a reader knows of an example, please let me know.

With today’s means it should be possible to break an autokey cryptogram without too much effort. For example, an autokey solver is available on the website dCode.

To check how difficult it is to decipher an autokey ciphertext I decided to create a challenge. The following cryptogram has been created with the autokey cipher:


The plaintext is in English. The keyword is probably not contained in any dictionary file. The dCode autokey solver didn’t break the cryptogram.

Can a reader decipher this message? If so, how did you do it?

Further reading: A crypto competition from 1875


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Kommentare (7)

  1. #1 Michael BGNC
    Lake Constance
    22. Januar 2020


    High blood Drumming on your skin it’s so tight You feel my heat I’m just a moment behind

  2. #2 Klaus Schmeh
    22. Januar 2020

    Great! This is correct.

  3. #3 Nils Kopal
    22. Januar 2020



    CrypTool 2 – Vigenere Solver needs 7 seconds to solve it 😉

  4. #4 Nils Kopal
    22. Januar 2020

    actually, Autokey is as easy to solve as Standard Vigenère 🙂

  5. #5 Nils Kopal
    22. Januar 2020

    Also, only “LTUKVYYZDVTBTUOUHZBMRXIMWUVVZGFMHCX” is enough to solve it 🙂

  6. #6 Klaus Schmeh
    22. Januar 2020

    @Nils: Thanks for the information.
    I checked if CrypTool has an autokey solver, but I didn’t find one. When I entered “autokey” into the search field, there was no result.

  7. #7 Nils
    22. Januar 2020

    It is “hidden” in the Vigenère solver… I add it as keywords to the template