1. Section 2.5 Polyaphabetic Substitutions
The monalphabetic ciphers (e.g. shift, affine, substitution) are vulnerable to frequency analysis.
To increase security, we can change the cipher alphabet as we are enciphering the message.
Example 1: Encipher the even letters according to one scheme and the odd letters to another…

2. Polyalphabetic Substitutions
Definition: A polyalphabetic cipher is a cipher where different substitution alphabets are used for various parts of the plaintext.
Four more famous versions of this are
The Alberti Cipher Disk
Thomas Jefferson Cipher Wheel
German Enigma Cipher Machine
Vigenere Cipher (we talk about this next)…

3. Vigenere Cipher
The Vigenere cipher is a polyalphabetic cipher invented by the Frenchman Blaise de Vigenere in 1585.
It was not widely used until the 1800’s when frequency analysis was used to break the simpler monoalphabetic ciphers.
Here is how it works:
Pick a keyword
Repeatedly write the keyword below the plaintext, one letter of the keyword associated with one letter of the plaintext.
Use the Vigenere Cipher Square to cipher the text. (See Mr. Fisher’s Program, also.)
The ciphertext letter is found by intersecting the row headed by the plaintext letter, and the column headed by the keyword letter.
Example 2: Vigenere cipher…

4. Decipher Vigenere Cipher
To decipher a Vigenere cipher:
On the second row write the ciphertext.
On the first row write the keyword repeatedly, one letter above each letter of the ciphertext.
Read off the ciphertext letter by: (Using Vigenere Square)
Using the keyword letter to find the row.
Find the ciphertext letter in that row.
Go to the top of the column which contains the ciphertext letter and read off the plaintext letter.
Example 3: Decipher a message…

5. Finding the Keyword
If you know the plaintext and the ciphertext you can get the keyword:
Find the plaintext letter across the top of the Vigenere Square.
Find the ciphertext letter in that column belonging to that letter.
The heading letter to the row containing the ciphertext letter is the keyword letter.
Example 4: Find six-letter keyword…!