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It's more a theoretical question of how would you approach it. All you know about the ciphertext it's was generated with a historical cipher. The ciphertext appears to be random, BUT it's divided into sets of six letters. By historical ciphers I mean: mono (Ceasar), polyalphabetic (Vigenere), homophonic or polygraphic ciphers (Playfair, HillCipher).

I interested what methods would you use (Kasiski, Friedman, what else) and why and in what order to make the attack the most efficient (Friedman first?)

Hendrik Brummermann
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Adam
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1 Answers1

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If you don't know the system, you just check one after the other:

  • frequency analysis of bigrams detects Ceasar and Playfair. Try Caesar first then Playfair.
  • Auto correlation method for Vigenere (for each x: count the number of occurances, where letter at position i and i+x are equal. For the correct codeword length, it will spike)
  • If you have a Hill cipher, you lost.
tylo
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