What is the Vigenère Cipher?

The Vigenère cipher is a polyalphabetic substitution cipher that is a natural evolution of the Caesar cipher. The Caesar cipher encrypts by shifting each letter in the plaintext up or down a certain number of places in the alphabet. If the message was right shifted by 4, each A would become E, and each S would become W.

In the Vigenère cipher, a message is encrypted using a secret key, as well as an encryption table (called a Vigenere square, Vigenere table, or tabula recta). The tabula recta typically contains the 26 letters of the Latin alphabet from A to Z along the top of each column, and repeated along the left side at the beginning of each row. Each row of the square has the 26 letters of the Latin alphabet, shifted one position to the right in a cyclic way as the rows progress downwards. Once B moves to the front, A moves down to the end. This continues for the entire square.

Also, other alphabets than the English alphabet can be used in a similar way to construct a tabula recta.

Let’s take this plaintext phrase as an example:

IMPROVE YOUR PUZZLE SOLVING SKILLS

After finalizing the plaintext, the person encrypting would then pick a secret key, which would help encrypt and decrypt the message. Our example secret key here is:

BOXENTRIQ

The next step is repeating the secret key enough times so its length matches the plain text.

IMPROVE YOUR PUZZLE SOLVING SKILLS
BOXENTR IQBO XENTRI QBOXENT RIQBOX

Once the two lines are split into five-letter groups, start encrypting. Take one letter from the plaintext group and a letter from the secret key group (we’re going to start with I and B), and find the entry in the tabula recta where the row and column intersect. For this example, the first letter of the encrypted cipher text is J.

Once you’ve done that for every character, your final encrypted text should look like this:

JAMVB OVGEV FMYMS CMIPZ SMAZJ SYMZP

You can use this cipher for short or long messages. Once you’ve mastered the tabula recta, the encryption process is easy!

How to Decrypt it

If you have the secret key, decrypting is as easy as encrypting. You can work backwards using the tabula recta. First repeat the secret key so its length matches the cipher text.

JAMVB OVGEV FMYMS CMIPZ SMAZJ SYMZP
BOXEN TRIQB OXENT RIQBO XENTR IQBOX

Using the tabula recta, find the row that corresponds to the first letter in your secret key text- in our case, B. In the B row, find the corresponding cipher text letter J. The vertical column where that cipher text letter is located reveals the plaintext letter I.

The Vigenère cipher can also be described and then decrypted algebraically, by assigning each letter from A to Z a value from 0 to 25, with addition being performed modulo 26.