Polyalphabetic Ciphers

• Vigenere (C = P + Ki mod 26)
• Key is K1, K2, ... Kn
• Example

  THEXCITYXISXATHENS
  KEYKEYKEYKEYKEYKEY
  DLCHGGDCVSWVKXFORQ
  

• Try to decode

  WEAOHMXMY
  

• Cryptanalysis
  • Determine index of coincidence (IC)
  • Measures variation of letter frequencies
  • Can be used to derive period p
  • From then on solve as p substitution ciphers

• PreviousSubstitution Ciphers
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 Last change: Friday, November 5, 2004 12:30 am
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