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:
- 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.
- If chosen by a human from a dictionary, I would also expect no harm from allowing repeated letters.
- 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?