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Cryptanalysis

Cryptanalysis. codes & ciphers. 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.

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Cryptanalysis

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  1. Cryptanalysis codes & ciphers

  2. 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

  3. 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

  4. Straddling Checkerboard

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

  6. 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

  7. Polyalphabetic Substitution Ciphers • Della Porta table

  8. 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 Open Vigenère table in Excel Open Rogue’s Gallery next Pictured below: key numbers used in Gronsfeld variation

  9. 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

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

  11. 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.

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

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