Vigenère Cipher Decoder
Encode and decode the Vigenère cipher — the classic keyword-driven polyalphabetic cipher. Type a keyword, paste your text, and switch between encode and decode. The keyword picks a different Caesar shift for every position, case is preserved, and non-letters pass through unchanged. Everything runs in your browser; your message and keyword are never uploaded.
Enter a keyword (letters) to encode or decode.
Runs entirely in your browser — the message and your keyword are never uploaded. The Vigenère cipher is a historical teaching cipher, not modern encryption; don't use it to protect anything that actually needs to stay secret.
What this tool is
- A free, in-browser Vigenère cipher tool: enter a keyword and encode or decode text with the classic repeating-key cipher.
- The keyword chooses a different Caesar shift for each position, so the same plaintext letter can encrypt to many different letters — this is what makes it polyalphabetic.
- Standard A–Z: upper and lower case are preserved, non-letters (spaces, digits, punctuation) pass through, and the key advances only over letters.
- Everything runs client-side in JavaScript — your message and keyword are never sent to a server.
- The Vigenère cipher is an educational classical cipher. It falls to Kasiski examination and frequency analysis and is NOT secure encryption.
How the Vigenère cipher works
The Vigenère cipher is a repeating-key cipher: it is really a stack of Caesar ciphers chosen by a keyword. You write the keyword repeatedly above the message, and each key letter gives the shift for the letter beneath it — A means shift 0, B shift 1, up to Z shift 25. For example the keyword KEY applied to HELLO shifts H by 10, E by 4, L by 24, L by 10, O by 4, producing RIJVS.
The details that matter
- Polyalphabetic. Because the shift changes from position to position, one plaintext letter can map to many cipher letters — flattening the single-letter frequency that gives a Caesar cipher away.
- The key cycles over letters only. Spaces, digits and punctuation pass straight through and do not consume a key letter, so the keyword stays aligned to the actual letters of the message.
- Decoding is the inverse. To decrypt, subtract each key letter's shift instead of adding it. Give this tool the same keyword and switch to Decode.
Why it eventually falls
The weakness is the repeating keyword. When the same word in the plaintext lines up with the same part of the key, it produces the same cipher text, and the gaps between those repeats are multiples of the key length. The Kasiski examination (and the index of coincidence) exploits this to recover the key length; once you know it, the cipher text splits into slices that are each a simple Caesar cipher, broken by ordinary frequency analysis. What took cryptanalysts centuries to work out, a computer now does in an instant.
Security note: The Vigenère cipher is an educational classical cipher, not encryption. It resisted hand cryptanalysis for a long time, but Kasiski examination and frequency analysis break it routinely today, so it must never protect passwords, messages or any sensitive data. Use it for learning, puzzles and games — and a modern authenticated cipher such as AES-GCM when you need real confidentiality.
Frequently asked questions
What is the Vigenère cipher?
The Vigenère cipher is a polyalphabetic substitution cipher from the 16th century. Instead of shifting every letter by the same amount like a Caesar cipher, it uses a keyword: each letter of the keyword specifies a shift, and the keyword repeats across the message. So the first letter is shifted by the first key letter, the second by the second, and so on, wrapping back to the start of the keyword when it runs out. Because the shift changes from position to position, it was once called "le chiffre indéchiffrable" — the indecipherable cipher.
How do I decode a Vigenère cipher?
If you know the keyword, decoding is just the reverse of encoding: each cipher letter is shifted back by the corresponding key letter. Switch this tool to Decode mode, enter the same keyword, paste the cipher text, and the plaintext appears. If you do not know the keyword, you first have to recover its length — usually with the Kasiski examination or the index of coincidence — and then solve each slice as a separate Caesar cipher using frequency analysis.
How is Vigenère different from the Caesar cipher?
A Caesar cipher uses one fixed shift for the whole message, so it has only 25 possible keys and the letter-frequency pattern of the language survives intact. The Vigenère cipher applies a sequence of Caesar shifts driven by a keyword, effectively interleaving several Caesar ciphers. This flattens the single-letter frequencies and vastly enlarges the key space, which is why Vigenère resisted casual cryptanalysis for centuries while a Caesar cipher can be broken by hand in seconds.
Is the Vigenère cipher secure?
No. Although it is far stronger than a Caesar cipher, the repeating keyword is its downfall. Charles Babbage and Friedrich Kasiski showed in the 19th century how to find the key length from repeated patterns in the cipher text, after which each position is just a Caesar cipher solvable by frequency analysis. Modern computers do this instantly. Vigenère is valuable for teaching the ideas behind polyalphabetic encryption and for puzzles, but it must never be used to protect real data — use a modern authenticated cipher such as AES-GCM for that.