Vigenère Cipher

11 June 25

The indecipherable cipher By W Bro Dr Max Katz-Barber

This method of encryption dates back to the 16th century and is named after Blaise de Vigenère, a French diplomat and cryptographer.

Originally devised by Giovan Battista in 1563, it was later improved and popularized by Vigenère in 1586. For centuries, the Vigenère cipher was considered unbreakable, as its complexity, especially with a long and random keyword, made it incredibly difficult to decipher without knowing that keyword.

Breaking the Cipher:

The Vigenère cipher’s security was eventually broken in 1863 by Charles Babbage (the same guy who invented the first computer) and Friedrich Kasiski – a German cryptanalyst. Kasiski discovered that repeated sequences of letters in the ciphertext could be used to deduce the length of the keyword. Once the length of the keyword was known, it could be broken down into simpler Caesar ciphers, making the overall message decipherable at last.

The Vigenère cipher laid the foundation for more complex encryption systems and is still an important historical cipher in the study of cryptography.

The Vigenère cipher is categorised as a ‘polyalphabetic substitution’ method, a similar concept to that used for the Enigma Machine during the Second World War. Its invention marked a significant step forward in cryptography, demonstrating the power of polyalphabetic ciphers and the importance of using a long, random key for encryption. It is still an important historical cipher in the study of cryptography.

Decoding a message

We’ll start with a short example, then you’ll get one for yourself.

To decode a message you will need the coded words and the keyword.

A handy table to have is the Alphabet Numerator shown in Figure 1.

EXAMPLE: See Figure 2 – Example. The coded message is CLV MU WOPN. The keyword is ‘CAKE’.

Step 1: Write out the coded message.

Step 2: In the line below it, write the keyword so that its letters align with the letters of the coded message above it. Keep repeating the keyword until you reach the end of the coded message. Do not write anything in for spaces or punctuations.

Step 3: Once the keyword has been aligned, see what letter on the keyword line aligns with each letter of the code. In this example, the first letter ‘C’ in the code, aligns with ’C’ in the keyword line.

Step 4: Look up ‘C’ in the Alphabet Numerator and you will see that it equates to ‘2’.

Step 5: Count back two steps from ‘C’ and you will get ‘A’. The first letter of the message is A.

Step 6: Do that for each letter of the code and you will see the decoded message ‘ALL IS WELL’. Got all that? Now try the technique out for yourself by solving the masonic cipher on page 45 of the latest Freemason 😉