Vigenère Cipher
Enter an ASCII-letter key and choose encryption or decryption. Only A–Z letters are shifted; every other character stays unchanged.
A QUICK WALKTHROUGH
How to use this tool
- Enter text and a nonempty key made of ASCII letters A–Z.
- Choose Encrypt or Decrypt.
- Review and copy the result; processing happens locally in your browser.
Classic Vigenère rules
Each key letter supplies a repeating shift from 0 to 25. Encryption adds the shift and decryption subtracts it. Uppercase and lowercase ASCII letters keep their original case.
Predictable text boundaries
The key must contain only ASCII letters A–Z or a–z. The key advances only when the input contains an ASCII letter, so spaces, punctuation, digits, accented letters, CJK text, and emoji do not consume key characters or change.
A local learning tool
This is a classical substitution cipher for learning and compatibility. It is not suitable for protecting passwords, secrets, or sensitive data. Text and keys stay in this browser.
GOOD TO KNOW
Common questions
Which characters are changed?
Only ASCII letters A–Z and a–z. Case is preserved, and all other characters remain unchanged.
Does punctuation advance the key?
No. The key advances only when an ASCII letter is processed.
Can I use accented or non-Latin letters in the key?
No. The key must be nonempty and contain ASCII letters only. Non-ASCII characters in the text remain unchanged.
Is Vigenère encryption secure?
No. It is a classical cipher and should not protect secrets. Use a modern, reviewed encryption system for sensitive data.