Repeating-key Vigenère and plaintext autokey can begin with the same word, use the same A=0 through Z=25 arithmetic, and encrypt the same plaintext. They stop behaving like the same cipher as soon as that starting word is exhausted. Take:
A repeating-key Vigenère cipher starts with a keyword and cycles that same keyword for as long as the message lasts. An autokey construction takes a different approach. It begins with a short secret primer , then extends the keystream using material from the message itself.
The number 26 is not built into the mathematical idea of Vigenère. It comes from one particular alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Standard Vigenère assumes one familiar ordering: ABCDEFGHIJKLMNOPQRSTUVWXYZ That ordering does more than display the letters.
A Vigenère cipher does not become mathematically different when the plaintext changes from English to Spanish, French, or German. The cryptanalysis can. Once a likely key period is known, a common attack splits the ciphertext into recurring columns and solves each column as a Caesar shift. The scori
A Vigenère implementation can produce convincing ciphertext and still be wrong. The arithmetic is only one part of the job. A correct program also has to decide:
A useful Vigenère cracker is not one formula. It is a pipeline. The ciphertext first has to be normalized. Plausible key periods have to be generated. Each candidate period creates a different set of Caesar-like columns. Those columns have to produce a key. Harmonic periods have to be recognized. Fi
Chi-squared scoring and n-gram scoring are often discussed as though they solve the same Vigenère problem. They do not. Chi-squared is naturally suited to the individual Caesar-shifted columns that appear after a Vigenère period is known.
A Vigenère cracker often reaches a stage where the hard question is no longer: Can I produce a decryption? It is:
Once the Vigenère key period is known, the ciphertext stops behaving like one complicated polyalphabetic stream. It can be split into columns. Each column was encrypted by one fixed Caesar shift.
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