Polygraphic Substitution ADFGVX Uses a random alphabet table

Polygraphic Substitution • ADFGVX – Uses a random alphabet table – Keyword in secondary grid to rearrange message • Nihilist cipher – Uses a Polybius square to change letters into numbers – Adds numbers derived from a keyword to create ciphertext

Polygraphic Substitution • Straddling checkerboard – Converts plaintext into digits – A monome-binome cipher • VIC – Uses a straddling checkerboard to obtain numerical values for letters – Adds a random number key (using modular arithmetic) – Optionally, converts these numbers back into letters using the straddling checkerboard to create the final ciphertext

Straddling Checkerboard 0 1 2 3 4 5 6 7 8 9 E T A O N R I S 2 B C D F G H J K L M 6 P Q / U V W X Y Z .

Polygraphic Substitution • Bifid cipher – Combines Polybius square with transposition – Writing numbers on two lines

Polygraphic Substitution • Trifid cipher – Similar to bifid cipher, except each letter is assigned a 3 -digit number using digits 1 -3 – Fractionation is accomplished by splitting the numbers onto three lines and then writing the resulting ciphertext letters

Polyalphabetic Substitution Ciphers • Della Porta table

Polyalphabetic Substitution Ciphers • Vigenère table – use columns to look up letters of the text – the key indicates which row to use to encipher or decipher the message – autokey improves security Pictured below: Open Vigenère table in Excel Open Rogue’s Gallery next key numbers used in Gronsfeld variation

Polyalphabetic Substitution Ciphers • Rogue’s gallery – Vigenère table with double columnar transposition – advantage is it creates a cipher that is tough to break – drawback is it is a complex operation, and prone to errors – recipient needs to know the keyword that was used to create the table and the keyword that was used to encipher the text

Modern Cryptography • Hill cipher – Uses matrices and modular arithmetic • One-way functions – Alice & Bob example

The Unbreakable Cipher • One-time pad is unbreakable if it meets the following conditions: – The key must be random. – The key is as long as the plaintext. – It must be used only once. – It must be kept secret from everyone except the sender and the intended receiver.

Unsolved Mysteries • • • Linear A Etruscan Iberian Phaistos Disk Bronze Age Indus Script Beale Ciphers Rongorongo Runic Ogham