Vigenère Cipher Decoder & Encoder
intermediatePolyalphabetic substitution using a repeating keyword
What is Vigenère Cipher?
The Vigenère cipher is a polyalphabetic substitution cipher that uses a repeating keyword to apply multiple different Caesar shifts to the plaintext. For centuries it was called "le chiffre indéchiffrable" (the indecipherable cipher) before Charles Babbage and Friedrich Kasiski independently broke it in the 19th century.
How Vigenère Cipher Works
Write your keyword repeatedly above the plaintext. For each letter, shift the plaintext letter by the alphabetical position of the keyword letter (A=0, B=1, C=2...). A keyword of "KEY" means K=10, E=4, Y=24, and these shifts repeat throughout the message.
Vigenère Cipher in Pop Culture
The Vigenère cipher appears in many spy novels and puzzle games. It's used in ARGs (alternate reality games), escape rooms, and has appeared in TV shows like "Gravity Falls" and "Prison Break."
How to Break Vigenère Cipher
The Kasiski examination and Index of Coincidence analysis can determine the key length. Once the key length is known, the message splits into several Caesar ciphers, each solvable by frequency analysis.
Frequently Asked Questions
How do I decode Vigenère Cipher?
Enter your ciphertext in the tool above, select "Vigenère Cipher" from the dropdown, choose Decode mode, enter the key, and click the button. Results appear instantly.
What is Vigenère Cipher used for?
Vigenère Cipher was used since 1553 for secret communication. It offers better security than simple ciphers and was used for serious communication.
How secure is Vigenère Cipher?
Vigenère Cipher provides moderate security against casual attacks but is vulnerable to dedicated cryptanalysis. It's suitable for historical study but not modern security.
Who invented Vigenère Cipher?
Vigenère Cipher was invented by Giovan Battista Bellaso around 1553.