Vigenere Cipher Tool
Encrypt or decrypt text with a keyword. Case and punctuation are preserved — only letters shift.
Worked example
Message HELLO with keyword KEY:
| Message | H | E | L | L | O |
|---|---|---|---|---|---|
| Keyword | K | E | Y | K | E |
| Cipher | R | I | J | V | S |
Each message letter is shifted forward by its keyword letter: H + K = R, E + E = I, L + Y = J, L + K = V, O + E = S. Decryption shifts each letter back by the same keyword.
How to use
- Choose Encrypt or Decrypt.
- Type your message and a keyword of letters only.
- Press the action button — the result appears below and you can copy it.
Note: the Vigenere cipher is a learning classic, not strong modern security. Do not use it for anything that must stay secret.
Frequently Asked Questions (FAQ)
What is the Vigenere cipher?
The Vigenere cipher is a classic polyalphabetic cipher: each letter of your message is shifted by the letter of a repeating keyword, so the same plain letter can encrypt to different cipher letters.
Does the tool change my capital letters?
No. Capitals stay capitals, small letters stay small, and spaces, numbers and punctuation pass through unchanged. Only letters shift, using a case-insensitive keyword.
What makes a good keyword?
Use a keyword of letters only. The receiver needs the exact same keyword to decrypt your message.
Can I both encrypt and decrypt with this tool?
Yes. Choose Encrypt or Decrypt, type the message and the same keyword of letters only, then press the action button — the result appears below and can be copied.
Is the Vigenere cipher secure for real secrets?
No. It is a learning classic, not strong modern security. Do not use it for anything that must stay secret.