Overview:

• Vigenère cipher was invented by Blaise de Vigenère in 1585.
• It is considered an example of a polyalphabetic substitution, where multiple Caesar (shift) ciphers are joined together.
• The code was considered unbroken for about 300 years.

The Vigenère Square:

Vigenère used the following square

The table has the following features:

• The top row and left most column contain the English alphabets.
• Each row in the table (excluding the top row) contains the alphabet with some shift.
  • The first row is shifted by 0, the second row by 1 and so forth.
• Each column also contains a shifted version of the alphabet, where column 1 has 0 shifts, column 2 has 1 shift and so forth.

The table is to be interpreted as the following:

• The top-most row is used for the plaintext characters.
• Left-most column is used for the key characters.
• All other columns and rows are used for the ciphertext.

Vigenère Cipher (Version 1: Autokey)

Encryption:

1. Both Alice and Bob agree on a single character to be the key (It is also known as the auto-key). Let it be the character: P
2. The sender writes the plaintext in one line. I Saw Michelangelo at work I S A W M I C H E L A N G E L O A T W O R K
3. The sender prepares another line (to be aligned with plaintext and written beneath it) which consists of the key + the plaintext (except the last character). I S A W M I C H E L A N G E L O A T W O R K P I S A W M I C H E L A N G E L O A T W O R