Polygraphic Substitution Portas digraphic substitution In this system

Polygraphic Substitution • Porta’s digraphic substitution – In this system, two plaintext letters will be represented by a 3 -digit number in the ciphertext.

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Polygraphic Substitution • Gemini – The message is converted into random digraphs, with 2 letters of ciphertext corresponding to 2 letters of the original message. – Disadvantage is the large table of letters to carry around

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Polygraphic Substitution • Polybius square – A single letter of the message produces two characters in ciphertext – Use of keyword or random alphabet improves security

Polybius Square 1 2 3 4 5 1 A B C D E 2 F G H I/J K 3 L M N O P 4 Q R S T U 5 V W X Y Z

Polygraphic Substitution • Playfair cipher – Uses a keyword – Special rules for selecting pairs of letters

Polygraphic Substitution • 2 -square cipher – Similar to Playfair, but using two random alphabet tables • 4 -square cipher – Uses 4 random alphabet tables

2 -Square Cipher

Other Polygraphic Substitution Ciphers • Vixen – Polygraphic substitution with homophones – 10 x 10 grid numbered 0 -9 – The quick brown fox jumps over the lazy dog. – The message is enciphered row x column. – The advantage is that it is easy to set up and one letter can be enciphered in several different ways, particularly the most common letters.

Other Polygraphic Substitution Ciphers • Calendar cipher – specify the month and year to the recipient – distinguish between Tuesday & Thursday, and Saturday & Sunday – Example: March 2014 W-1 would be the first Wednesday The date is the 5 th, so it corresponds to the 5 th letter in the alphabet = E.

Other Polygraphic Substitution Ciphers • Coordinate system – divide the alphabet into 4 quadrants – use points (x, y) to encipher each letter – points that do not correspond to letters are used as nulls

Other Polygraphic Substitution Ciphers • Some ciphers are simpler than they seem. For example, the middle letter in each set of 3 letters could be shifted x spaces forward to reveal the plaintext. Only the middle letters in each group are significant. The rest are nulls. • Another possibility is adding all the numbers in each 3 -digit sequence to get a number corresponding to an alphabet letter, or perhaps a reference to its location on a Polybius square or straddling checkerboard.