Vigenère Cipher
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Definition
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a 16th-century cipher (Vigenère) where the key is a short word, repeated to match the message length, and each letter is combined with the corresponding key letter via addition mod 26 (decryption subtracts mod 26)
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Why it's a step up from simple substitution
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the same plaintext letter can map to different ciphertext letters depending on position — defeating naive single-letter frequency analysis directly
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Breaking it (assuming the key length is known)
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split the ciphertext into groups by position mod (key length) — every letter in one group was shifted by the same key letter
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within each group, the most frequent ciphertext letter is very likely the encryption of E; subtracting E's frequency rank recovers that one key letter; repeat per group to recover the whole key
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Breaking it (key length unknown)
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simply try key length 1, then 2, then 3, ... running the above attack each time, until one length produces a decryption that actually reads as sensible text
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The good idea, poorly executed
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addition mod 26 (rather than simple substitution) is actually a sound idea — modern ciphers use essentially the same idea (XOR/addition with a keystream) — Vigenère's mistake was using a short, repeating key rather than a key as long as the message, or a pseudorandom keystream; see One-Time Pad and Stream Cipher
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