Tools / Vigenère Cipher
Vigenère Cipher
Encrypt or decrypt text with the Vigenère polyalphabetic substitution cipher. Enter the key (letters only) and the ciphertext to recover the plaintext. Non-letter characters pass through unchanged.
Only letters are used; digits and symbols are ignored. Repeats cyclically.
Paste text and enter the key to get started.
How the Vigenère cipher works
Unlike a simple Caesar cipher that shifts every letter by the same amount, the Vigenère cipher uses a repeating keyword. Each letter of the key determines the shift for the corresponding letter in the message: A=0, B=1, …, Z=25. This makes frequency analysis harder because the same plaintext letter can map to different ciphertext letters depending on its position.
Decryption is the reverse: subtract the key letter's shift (mod 26) from each ciphertext letter. The key repeats cyclically until the message is fully processed.
Challenges solved with this tool: picoCTF 2022 -- Vigenere.