Vigenere Cipher

How to use

1. Enter Text — Type the message you want to encrypt or decrypt.
2. Provide a Key — Enter a keyword or phrase. Only letters A-Z are used; spaces and numbers are ignored in the key.
3. Choose Mode — Select Encrypt or Decrypt. The same key is used for both directions.
4. Copy Result — The output updates live. Click Copy to use the ciphertext or plaintext.

Free Vigenere Cipher Encrypt & Decrypt

Encrypt and decrypt messages with the classic Vigenere cipher. Enter your text and a keyword to produce polyalphabetic substitution ciphertext. All processing is local and private.

Common Uses

• Learn historical cryptography techniques
• Create puzzles and scavenger-hunt clues
• Obfuscate casual messages for fun
• Teach substitution cipher concepts
• Compare strength against Caesar ciphers

How It Works

Each letter of the input is shifted by the corresponding letter of the key (A=0, B=1, ..., Z=25). The key repeats as needed. Non-alphabetic characters pass through unchanged. Because the shift changes with every letter, simple frequency analysis is ineffective.

Privacy

No messages are transmitted. Everything runs in your browser.

Related: ROT13 Encoder, Base64 Encoder/Decoder, Password Generator.

Frequently asked questions

What is the Vigenere cipher?

The Vigenere cipher is a polyalphabetic substitution cipher invented in the 16th century. It uses a keyword to shift letters by varying amounts, making it much harder to break than simple Caesar ciphers.

Is Vigenere secure today?

No. While stronger than Caesar ciphers, the Vigenere cipher is easily broken with modern cryptanalysis (e.g., Kasiski examination or Friedman tests). It is for educational and novelty use only.

What happens to numbers and symbols?

Numbers, spaces, punctuation, and other non-alphabetic characters are passed through unchanged. Only A-Z and a-z are transformed.

Does the key need to be a real word?

No. Any sequence of letters works. Longer and more random keys produce stronger obfuscation, though the cipher remains fundamentally breakable.

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