Encode and decode text using the Vigenère cipher, a classic polyalphabetic substitution cipher. Enter a keyword to shift each letter differently. Pure client-side, no data leaves your browser.
The Vigenère cipher is a method of encrypting alphabetic text using a simple form of polyalphabetic substitution. It was invented by Giovan Battista Bellaso in 1553 and later misattributed to Blaise de Vigenère. Each letter of the plaintext is shifted along some number of places determined by the keyword, making it much more secure than simple Caesar ciphers.
The Vigenère cipher uses a keyword to determine the shift for each letter. Each letter of the keyword corresponds to a shift value (A=0, B=1, ..., Z=25). The keyword is repeated to match the length of the plaintext. Each plaintext letter is shifted by the corresponding keyword letter value.
By modern standards, the Vigenère cipher is not secure. It can be broken using statistical analysis (Kasiski examination) if the keyword is short or repeated. However, with a truly random keyword as long as the message (one-time pad), it becomes theoretically unbreakable.
A Caesar cipher uses a single fixed shift for the entire message. The Vigenère cipher uses a different shift for each letter based on the keyword, making frequency analysis much more difficult.
This tool only uses alphabetic characters (A-Z, a-z) in the keyword. Non-alphabetic characters are ignored when determining shifts, but they are preserved in the output.
The cipher was actually invented by Giovan Battista Bellaso in 1553, but was later misattributed to Blaise de Vigenère, a French diplomat and cryptographer, in the 19th century.