Question: Given a long ciphertext encrypted by using the Vigenre cipher, the first step of the cryptanalysis involves working out the length of key. Briefly explain

Given a long ciphertext encrypted by using the Vigenre cipher, the first step of the cryptanalysis involves working out the length of key.

Briefly explain how to apply the Kasiski method to guess the likely lengths of the key.

Briefly explain what the index of coincidence is and how to apply it to determine the key length.

Assume you have obtained the key length by using either of the above methods. Explain how you can subsequently derive the key.

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