Vigenere Cipher

The Vigenere cipher is a classic polyalphabetic substitution cipher that improves on the simple Caesar shift by using a repeating keyword instead of a single fixed offset.

Each letter of your message is shifted by an amount taken from the matching letter of the keyword, so the same plaintext letter can encrypt to different ciphertext letters depending on its position.

This made the cipher famously hard to break by hand for centuries, and it is still a wonderful way to learn how key-based encryption works.

This tool encrypts and decrypts in both directions.

Pick a mode, type a keyword, and paste your text; the result updates instantly.

Only the letters A–Z of your keyword are used, so spaces and punctuation in the key are ignored.

In the message itself, case is preserved and any non-letter — digits, spaces, punctuation — passes straight through without consuming a key letter, which keeps the output readable and reversible.

Everything happens locally in your browser, so your messages and keys are never uploaded or stored.

Remember that the Vigenere cipher is a learning and puzzle tool, not modern security: it is easily broken by frequency analysis once the key length is known, so never use it to protect anything that genuinely needs to stay secret.