The Vigenère cipher
Encryption with a repeated key, decryption, finding the key length with the Kasiski test and the index of coincidence, and the return to several Caesar ciphers.
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Vigenère cipher: what is it?
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A cipher where a key, a word repeated beneath the message, gives for each letter the shift to apply modulo 26 (A = 0, B = 1…).
With key BAC, the first letter is shifted by 1, the second by 0, the third by 2, then it starts again: SALUT becomes TANVT.
With key ROI, ATTAQUE becomes RHBRECV. Why do the two Ts not give the same letter?
- They are shifted by O, then by I
- The cipher changes key at each doubled letter
- The two Ts have different ranks in the alphabet
- The ciphertext keeps a calculation error
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They are shifted by O, then by I
The key letters follow R, O, I, R, O, I, R: the first T falls under O (+14, so H), the second under I (+8, so B). This is the difference from Caesar.
In Vigenère, the key letter … shifts nothing, and the letter … shifts by one place.
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In Vigenère, the key letter A shifts nothing, and the letter B shifts by one place.
The alphabet is numbered from 0: A = 0, B = 1, C = 2… The key AAA leaves the message as it is.
Polyalphabetic substitution: what is it?
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Encryption where a single plaintext letter can be replaced by different letters depending on its position in the message.
With key BA, ELLE becomes FLME: the two Ls give L then M. This blurs letter frequencies, but the repetition of the key leaves an exploitable trace.