Unicity Distance for English

• English: H(M)=2.9 bits per character (symbol)

  • Actually less if you include spaces...

• For a random key, H(K)=4.7 bits per character

• H(K)/(|M|-H(M)) => |K|*4.7/(4.7*|M|-2.9*|M|)

  • |K| characters of key, |M| characters of message
  • English content codes are breakable when 4.7/1.8*|K|<|M|, OR 2.6*|K|<|M|
  • In other words, if the message is 2.6 times the size of the key (or greater), the code is breakable
  • given enough time

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