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A theoretical question about the Vigenère cipher:

  1. Without any knowledge about the key (not even it's length) can we tell how much known or chosen plaintext is needed for the adversary to completely recover the key?
  2. What about if we know the key length? (I assume that we'll need plaintext with the same length of the key).
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1 Answer

up vote 2 down vote accepted

For chosen plaintext and a classic Vigenère cipher, you need only as many characters of plaintext as the length of the key to completely recover the key. It's a trivial reversal. (If the cipher uses 26 scrambled alphabets, it will take more.) if you don't know the length of the key, you will spot the repeating sequence after the second repeat.

For an unknown ciphertext, that's when it gets interesting. You need enough repeats of the keylength to measure the index of coincidence. The Friedman test provides a way to estimate the amount of ciphertext.

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