Sunday, September 22, 2013

Notes - Substitution Ciphers

The following are notes from Introduction to Cryptography with Coding Theory.
  • Shift and Affine ciphers are examples of substitution ciphers Vigenere and Hill Ciphers are not
  • Substitution Ciphers can be broken by a frequency count but how exactly does the process go?
    • Approximate Frequencies of letters in english
      • e - 0.127
      • t - 0.091
      • a - 0.082
      • o - 0.075
      • i - 0.070
      • n - 0.067
      • s - 0.063
      • h - 0.061
      • r - 0.060
    • With the exception of E the other letters are close enough that for a small sample we would not be able to decide which is which therefore we need to look at pairs of letters
      • e often contacts many low-frequency characters
      • a, i, o tend to avoid one other
      • n has around 80% chase to be preceded with vowels
      • h often appears before e and rarely after it
      • most common digram is th
      • r pairs more with vowels than s among the frequent letters
      • combination rn should appear more than nr
      • to is more common than ot
    • Proceeding applying these rules, it is necessary to decrypt the message through recognizing partial words within the text to figure out the key for low frequency letters

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