Classical Monoalphabetic Ciphers

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Classical Monoalphabetic Ciphers

• Day 2

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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.

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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.

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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.

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Compile class data
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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.

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Frequency analysis

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

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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

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Affine example
Note a a-1 1 (mod n)

• a 3, b 7
• Find the equations for encryption and decryption.
• Encrypt the message the dog
• Decrypt the message TIVUJWL

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Affine cryptanalysis

• Is an affine cipher easier or harder to break
  then a keyword cipher?
• How would we break an affine cipher?

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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.

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Multiliteral cryptanalysis

• Is an multiliteral cipher easier or harder to
  break then a keyword cipher?
• How would we break an multiliteral cipher?

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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.

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Monoalphabetic cipher history

• Middle Ages nomenclator
• Monoalphabetic and some substitutions for words
  or phrases
• Mary, Queen of Scots

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Review