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Classical Ciphers Exploration: History, Analysis & Cryptanalysis

Explore classical ciphers like keyword, affine, and multiliteral with VBA code examples, frequency analysis, and historical insights. Learn techniques for encryption, decryption, and breaking these ciphers.

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Classical Ciphers Exploration: History, Analysis & Cryptanalysis

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  1. Classical Monoalphabetic Ciphers Day 2

  2. Keyword cipher • Select a keyword, if the keyword has any repeated letters, drop all but the first occurrence. • Write the keyword below the alphabet and fill in the rest of the space with the remaining letters of the alphabet in their standard order. • VBA Code • Modification: Allow the keyword to start anywhere along the alphabet.

  3. Keyword: cryptanalysis • Dictionary attach using a computer. • Every letter of a language has a personality of its own. • Determine the personality of each character in the ciphertext and try to match them with the known personalities of corresponding plain text. • This attach was used as early as the 9th century, by Arab scientist and philosopher al-Kindi.

  4. Frequency analysis • Assignment: • Download “Frequency_Analysis.xls” from the SMA website. • Find a 200-500 word section of common English writing. Copy and format it in Word. • Copy it. • Use it in the Excel spreadsheet to do a single, double, and triple letter frequency count.

  5. Compile class data

  6. Frequency analysis • “r” forms digrams with more different letters more often than any other letter. • The three vowles, “a”, “I”, and “o” avoid each other, except for “io”. • “ea” is the most frequent digram involving vowels. • Eight percent of the letters that procede “n” are vowels. • “h” frequently appears before “e” and almost never after it.

  7. Frequency analysis • Be willing to give up on an assumption and try something else if it appears that you are on the wrong path.

  8. Affine cipher • Each letter is assigned a number. “a” = 0, “b” = 1, “c” = 2, … • The key to an affine cipher is a pair of numbers (a, b). • The greatest common divisor (GCD) of a and 26 must be 1. • Let p be the number of the plaintext letter and c the number of the ciphertext letter. • c = (a p + b) mod 26 • p = (a-1(c – b)) mod 26

  9. Note: a a-1 = 1 (mod n) Affine: example • a = 3, b = 7 • Find the equations for encryption and decryption. • Encrypt the message “the dog” • Decrypt the message “TIVUJWL”

  10. Affine: cryptanalysis • Is an affine cipher easier or harder to break then a keyword cipher? • How would we break an affine cipher?

  11. Multiliteral cipher • It replaces each plaintext letter with a pair of letters. • Choose a 5 letter keyword with no repeating letters. • “i” and “j” occupy the same cell.

  12. Multiliteral: cryptanalysis • Is an multiliteral cipher easier or harder to break then a keyword cipher? • How would we break an multiliteral cipher?

  13. Monoalphabetic cipher history • The Argentis worked for the Pope during the late 1500 and early 1600s. • Probably first to use keyword • Numbers were used instead of letters • Used by the South during the Civil War.

  14. Monoalphabetic cipher history • Middle Ages – nomenclator • Monoalphabetic and some substitutions for words or phrases • Mary, Queen of Scots

  15. Review

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