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Cryptanalysis

Cryptanalysis. codes & ciphers. Modular arithmetic. Modular arithmetic limits the set of numbers used. Imagine a clock. How many hours are there between 7:00 AM and 3:00 PM? Examples: 2 + 6 (mod 7) = 1 11 x 9 (mod 13) = 8 * * because 99 divided by 13 is 7, remainder 8.

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Cryptanalysis

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  1. Cryptanalysis codes & ciphers

  2. Modular arithmetic • Modular arithmetic limits the set of numbers used. • Imagine a clock. How many hours are there between 7:00 AM and 3:00 PM? • Examples: 2 + 6 (mod 7) = 1 11 x 9 (mod 13) = 8 * * because 99 divided by 13 is 7, remainder 8

  3. Polygraphic Substitution Numeric substitution replace each letter with a number corresponding to its place in the alphabet (A=1, etc.)

  4. Polygraphic Substitution • Nifty number cipher • vowels are numbered • for consonants, count the # of spaces from the previous vowel in the alphabet • Example: MISTY = 3.4, 3.0, 4.4, 4.5, 5.4 • Jolly Roger • write the alphabet in 5 columns (Z not used) • letters A through E are listed as A-0, A-1, etc. • letters F through J are listed as B-0, B-1, etc. • to indicate the letter B, use A-1 or the letter A followed by one other letter in ciphertext • BAD WOLF is A-1, A-0, A-3, E-2, C-4, C-1, B-0 • This could be enciphered as AT A AQXL EVN CZQTR CW B.

  5. Polygraphic Substitution • Morse code • Binary system • ASCII • Bacon’s alphabet

  6. Morse Code Bacon's Alphabet

  7. More on Polygraphic Substitution Homophonic substitution • each character in the plaintext can correspond to several different ciphertext symbols • null symbols may be included • flattens frequency distribution, making analysis more difficult

  8. More on Polygraphic Substitution • Nomenclator • a homophonic substitution cipher with code words included

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