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… 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)… 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… 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… 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…!

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# Polyalphabetic Substitutions