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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