The Vigenere Cipher is a polyalphabetic substitution cipher that has been used in various forms since the 16th century. Unlike the Caesar cipher, which uses a single fixed shift for every letter, the Vigenere cipher applies a different shift to each letter based on a keyword. This makes the encoded text far harder to break with simple frequency analysis, because the same plaintext letter can appear as different ciphertext letters depending on its position in the message.
Here is how it works: the keyword is repeated over the plaintext so that each letter of the message has a corresponding keyword letter. Both letters are converted to numbers (A=0, B=1, ... Z=25), and the plaintext letter's value is shifted forward by the keyword letter's value. For example, if the plaintext letter is H (7) and the keyword letter is L (11), the result is S (18) because 7 + 11 = 18. To decrypt, the shift is simply reversed: subtract the keyword value instead of adding it.
Non-alphabetic characters like spaces, digits, punctuation are left untouched by default, so the structure of your message stays intact. The keyword itself should contain only letters; any numbers or symbols you type are stripped out automatically. Casing is preserved by default: an uppercase plaintext letter paired with a keyword letter produces an uppercase result, and lowercase stays lowercase.
This free tool runs entirely in your browser. Your text and keyword never leave your device, so you can safely use it for sensitive messages, classroom exercises, or puzzle-solving without worrying about where your data goes.