Vigenère Cipher: Polyalphabetic Encoding
Encode and decode Vigenère cipher messages — with keyword-based encryption and frequency analysis.
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Vigenère Cipher: Polyalphabetic Encoding Part of our complete guide to this topic — see the full series. The Vigenère cipher uses a keyword to apply a different Caesar shift to each letter of a message. It defeated simple frequency analysis for three centuries — until Kasiski and Babbage independently cracked it in the 1800s. --- All the tools discussed here are available for free at theproductguy.in — client-side, no sign-up required. How the Vigenère Cipher Works To encode a message, you align the plaintext with a repeating keyword and shift each plaintext letter by the alphabetical position of the corresponding keyword letter (A = 0, B = 1, … Z = 25). Example: Plaintext , keyword Each plaintext encodes differently depending on which keyword letter aligns with it: + = , + = , + = . The…
Frequently Asked Questions
What is the Vigenère cipher?
The Vigenère cipher is a polyalphabetic substitution cipher that uses a repeating keyword to shift each letter of the plaintext by a different amount. Each letter of the keyword specifies a Caesar shift for the corresponding letter of the message. The keyword repeats for the full length of the message.
How does the Vigenère cipher differ from Caesar cipher?
Caesar uses a single fixed shift for every letter. Vigenère uses a different shift for each letter, determined by the keyword. This prevents simple frequency analysis because the same plaintext letter can encrypt to different ciphertext letters depending on where it appears relative to the keyword.
How do I break a Vigenère cipher?
The standard approach has two steps. First, determine the key length using the Kasiski test (look for repeating sequences in the ciphertext) or by computing the index of coincidence for subsequences at different spacings. Then, treat each column of letters (those encrypted with the same keyword letter) as an independent Caesar cipher and apply frequency analysis.
What is the Kasiski test?
The Kasiski test finds the key length by locating repeated sequences of 3 or more characters in the ciphertext and measuring the distances between them. If the keyword length is L, repeated plaintext sequences that align with repeated keyword positions will appear at multiples of L. The GCD of the distances gives strong evidence for L.
What is an index of coincidence?
The index of coincidence (IC) measures how unevenly letters are distributed in a text. English text has IC ≈ 0.065; random text has IC ≈ 0.038. For a Vigenère ciphertext, splitting the text into every-L-th-character groups and computing their IC reveals the key length — the correct L gives groups with IC close to 0.065.
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