Encrypt or decrypt with a repeating keyword, the classic polyalphabetic cipher. Historically important, breakable since the 1860s, and educational only.
Show calculation steps
Processed privately in your browser — nothing you paste is uploaded, logged or stored.
A shift that changes each letter
Vigenère applies a Caesar shift that varies according to a repeating keyword, so the same plaintext letter encrypts differently depending on its position. That defeats simple frequency analysis and is why it was considered unbreakable for three centuries.
Worked example
plaintext ATTACKATDAWN key LEMONLEMONLE ciphertext LXFOPVEFRNHR The two As at positions 1 and 4 become L and O.
How it was broken
The keyword repeats, so the cipher is really several Caesar ciphers interleaved. Kasiski examination finds the key length by measuring the distance between repeated ciphertext sequences; once the length is known, each position is a Caesar cipher and falls to frequency analysis.
The one secure variant
A key as long as the message, truly random, and never reused is a one-time pad, which is information-theoretically unbreakable. Every one of those conditions is required; reusing a pad once destroys the guarantee entirely.
Do not use it for real work
A repeating-key Vigenère is broken by well-known methods and by software in moments. Use a vetted modern algorithm for anything that matters. Transformation runs in your browser.
How to use the Vigenere Cipher Encoder and Decoder
- Paste your message in the first panel.
- Enter a keyword in the second panel – it repeats across the message.
- Choose encrypt or decrypt, and whether punctuation advances the key.
- Click "Apply Vigenère".
Frequently asked questions
How does the Vigenère cipher work?
Each letter of the key selects a different Caesar shift, and the key repeats across the message. Because the same plaintext letter encrypts differently depending on its position, simple frequency analysis fails – which earned it the nickname "le chiffre indéchiffrable" for three centuries.
How was it eventually broken?
Charles Babbage and later Friedrich Kasiski showed that repeated sequences in the ciphertext reveal the key length, because they occur at multiples of it. Once the key length is known, the message splits into that many separate Caesar ciphers, each trivially broken by frequency analysis.
Does a longer key make it secure?
Only in the limiting case. A truly random key as long as the message, never reused, is the one-time pad – which is provably secure but impractical for most purposes. Any repeating key is vulnerable to the analysis above.
What does the punctuation option change?
Whether spaces and punctuation consume a key letter. Classical practice strips them entirely; keeping them and not advancing the key preserves readability of the layout while matching most textbook examples.
Break or check a simple cipher
Plaintext from a classical cipher, or confirmation of which one was used.
- ROT13 Encoder and DecoderTry ROT13 first: it is its own inverse, so one pass tells you immediately.
- Caesar Cipher Encoder and DecoderTry every Caesar shift, since ROT13 is only the shift-13 case.
- Atbash CipherTry Atbash, which reverses the alphabet rather than rotating it.
- Vigenere Cipher Encoder and Decoder you are hereIf a single shift never works, the key is probably a word rather than a number.
