Toolgeist › Security
Vigenère Cipher — Polyalphabetic Encode/Decode With a Keyword
Encode or decode text using the Vigenère cipher — a polyalphabetic substitution that shifts each letter by an amount determined by a repeating keyword, rather than a single fixed shift like Caesar. For roughly three centuries it was considered unbreakable ("le chiffre indéchiffrable"), until Friedrich Kasiski published a practical attack on it in 1863.
How to use Vigenère Cipher
- Paste your text.
- Enter your keyword.
- Read the encoded or decoded result.
Frequently asked questions
- How is this different from a Caesar cipher?
- Caesar shifts every letter by the same fixed amount; Vigenère shifts each letter by an amount determined by the corresponding letter of a repeating keyword, so the same plaintext letter can encode to different ciphertext letters depending on position.
- Why was it called "unbreakable" for so long?
- Because it resists simple frequency analysis, the technique that easily breaks a Caesar cipher — the same plaintext letter maps to different ciphertext letters depending on keyword position, which flattens the letter-frequency pattern attackers rely on.
- How was it eventually broken?
- The Kasiski examination (1863) exploits repeated sequences in the ciphertext to deduce the keyword length, after which the cipher reduces to several separate Caesar ciphers that frequency analysis can solve individually.
Related Security tools
Also searched for: vigenere cipher encoder decoder · vigenere cipher with keyword online · polyalphabetic cipher tool
Browse all 738 free tools on Toolgeist →