Vigenère Cipher

How to Use

1. Enter the text you want to encrypt or decrypt
2. Enter a key (letters only; the key repeats cyclically)
3. Click "Encode" or "Decode"
4. Click "Copy" to copy the result

How It Works

The Vigenère cipher is a polyalphabetic substitution cipher. Each letter in the key determines the shift for the corresponding plaintext letter (A=0, B=1, ..., Z=25).

Example (key = KEY):

Plaintext: H E L L O
Key:       K E Y K E
Shift:     10 4 24 10 4
Ciphertext: R I J V S

常见问题

How is the Vigenère cipher related to the Caesar cipher?

The Caesar cipher is a special case of the Vigenère cipher where the key is a single letter. The Vigenère cipher uses multiple letters as a key, combining multiple Caesar ciphers.

Is the Vigenère cipher secure?

It was called 'the indecipherable cipher' in the 16th century, but was broken in the 19th century using the Kasiski examination and index of coincidence. It's not suitable for protecting sensitive data.

How should I choose a key?

The longer and more random the key, the better. Avoid common words or phrases. Ideally, the key should be as long as the plaintext and consist of random letters.