Conceptindex of coincidence / 重合指数
Index of Coincidence
A statistical method used in cryptanalysis that measures the probability of coinciding letters in ciphertext to infer key length and to distinguish monoalphabetic from polyalphabetic substitution ciphers. It is commonly used to break the Vigenère cipher.
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Oct 14
The weakness of the Vigenère cipher is that when the key length is short and the ciphertext is long enough, an attacker can infer the key length through the Kasiski test or the index of coincidence, thereby reducing the polyalphabetic problem to multiple single-alphabet problems to crack one by one.
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