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I am studying the Vigenère Cipher that given a passphrase uses rotating Caeser Ciphers to encode the plaintext. The ‘classical’ attacks on Vigenère first deduce the length of the passphrase by conducting the Kasiski test and/or analyzing the index of coincidence.

To come to my initial question regarding repeated letters in the key. I guess the answer depends:

  1. If the key is chosen at random, repeated letters yield a larger keyspace at the same key length. Thus, allowing letters to repeat should strengthen the cipher.
  2. If chosen by a human from a dictionary, I would also expect no harm from allowing repeated letters.
  3. If however repeating letters yields repetitions of substrings in the key (e.g. ‘BABABA’), then the effective key length is reduced and thus the cipher is weakened.

Am I missing something here?

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  • The Kasiski test is useful to find the length, if the attackers know that it is Vigenère Cipher, they can try any length after the failure of Kasiski's result. 3) the key length 6 for you, 2 for the attackers. – kelalaka Jan 23 '20 at 14:07
  • @kelalaka, that's what I thought. Of course, if we were to choose ‘BABABX’ as a key. I'd suspect those heuristics to return a key length of 2. Which is false, but one should be able to recover (most) of the plaintext. Thus, the encryption is weakened by the repeated characters. – tim6her Jan 23 '20 at 18:45
  • There are online tools for Kasiski test. Why don't you try one? – kelalaka Jan 23 '20 at 18:55

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