Vigenere Cipher
Encrypt and decrypt text with the classic Vigenere cipher using a keyword. A polyalphabetic substitution cipher for learning and puzzles.
Last reviewed by the Radiatus Cloud team
Encrypt and decrypt text with the classic Vigenere keyword cipher.
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Encrypt with the Vigenere cipher
The Vigenere cipher is a classic polyalphabetic substitution cipher that encrypts text using a repeating keyword. Each letter of the message is shifted by an amount determined by the corresponding letter of the keyword, which repeats across the text. This makes it much stronger than a simple Caesar shift, because the same letter in the message can be encrypted differently depending on its position. Encrypting HELLO with the key KEY produces RIJVS.
Decryption reverses the shifts using the same keyword, and non-letter characters are passed through unchanged.
A piece of cipher history
For centuries the Vigenere cipher was considered unbreakable and was nicknamed the indecipherable cipher, until techniques for finding the key length broke it in the nineteenth century. Today it is a favourite for teaching cryptography concepts, for puzzles and geocaching clues, and for understanding why key reuse and short keys are weaknesses. It is not secure by modern standards and must not protect real secrets.
A longer, more random keyword makes the cipher harder to break, while a short or repetitive key is weak. For genuine security, use modern encryption. All processing happens locally in your browser.
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Frequently Asked Questions
How does the Vigenere cipher work?
Each letter is shifted by the value of the corresponding keyword letter, with the keyword repeating across the message, giving a polyalphabetic cipher.
Why is it stronger than a Caesar cipher?
Because the shift varies with the keyword, the same plaintext letter can encrypt to different letters, hiding the frequency patterns a Caesar cipher exposes.
Is the Vigenere cipher secure?
No. It was broken in the nineteenth century and is unsuitable for real secrets. It is useful for learning and puzzles only.
What makes a good keyword?
A long, random keyword with no repetition is hardest to break, while short or repeated keys weaken the cipher considerably.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter text and a keyword, then encrypt or decrypt.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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SecurityGenerate MD5, SHA-1, SHA-256 and SHA-512 hashes in your browser. Compare checksums and verify file integrity with nothing uploaded to a server.