Vigenère Cipher
With this tool you can use the Vigenère cipher: encrypt or decrypt text with a keyword, choose a variant and estimate an unknown key by analysis, all locally in your browser.
Source: Wikipedia – Vigenère-Chiffre (Verfahren, Tabula recta, Beispiel LEMON). Updated: .
How it is calculated
With the keyword LEMON, ATTACKATDAWN becomes LXFOPVEFRNHR. The Vigenère cipher shifts every letter by a different amount, given by the matching letter of the keyword (A = 0, B = 1 … Z = 25).
How it is calculated
To encrypt, plaintext letter plus key letter is taken modulo 26; to decrypt it is subtracted. The keyword repeats. In the Beaufort variant C = K − P, so encrypting and decrypting are the same calculation. In Autokey the key continues with the plaintext itself. The analysis finds the key length from the index of coincidence (Friedman) and each key letter from a chi-squared comparison with the letter frequencies of the chosen language.
Good to know
- For centuries the cipher was called “le chiffre indéchiffrable”. Friedrich Kasiski (1863) and William Friedman (1920s) showed it can be broken once the text is long enough.
- It is not suitable for real secrecy. Use modern encryption for that; the Vigenère cipher is a learning and puzzle tool.
- Umlauts become AE, OE, UE and ß becomes SS; digits and punctuation stay as they are and do not use up a key letter.
- Your input stays in the browser.
Frequently asked questions
How does the Vigenère cipher work?
Each plaintext letter is shifted along the alphabet by the amount given by the matching keyword letter (A = 0, B = 1 …). The keyword repeats until the text ends.
How do you decrypt a Vigenère cipher without the key?
First estimate the key length (Kasiski test or index of coincidence), then break each column like a Caesar cipher using frequency analysis. That is exactly what the “Find the key” action does.
What is the difference between Vigenère and Caesar?
Caesar shifts all letters by the same amount, Vigenère uses several shifts according to a keyword, which hides letter frequencies better.
Is the Vigenère cipher secure?
No. With enough text and frequency analysis a computer breaks it in seconds. It is not suitable for protecting real secrets.
Sources and legal basis
- Wikipedia – Vigenère-Chiffre (Verfahren, Tabula recta, Beispiel LEMON)
- Wikipedia – Autokey cipher (example KILT) and Beaufort cipher
- Wikipedia – Index of coincidence (Friedman test) and Letter frequency (language tables)
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