Vigenere Cipher Encoder & Decoder
Apply a repeating keyword, then audit every changed or preserved character in steps tied to your own result. Historically, the name is also spelled Vigenère.
Tap or place the cursor on a result character to focus its matching step.
Letter-by-letter steps
Each row comes from this result. A=0; preserved characters do not consume the keyword.
Processed locally in your browser. ToolNeko does not send your text, keyword, result, or export to a calculation service. Copy writes only the result to your clipboard; Download saves your input, result, and selected options to your device. Analytics records fixed action labels—not your values.
How the repeating keyword works
Write the keyword repeatedly under the ASCII A–Z letters in the message. Punctuation and other preserved characters do not consume a keyword position. The tool keeps source capitalization and uses one transformation pass for the result, trace, copy and export.
Read the letter-by-letter calculation
Each A–Z letter is numbered from A=0 through Z=25. Encoding adds the source and keyword indices modulo 26. Decoding subtracts the keyword index and wraps within 0–25. Focus a result character or a trace row to connect it to the exact source position.
Know the alphabet boundary
This page deliberately does not normalize, remove or transliterate characters. Accented letters, non-Latin scripts, emoji, digits, spaces, punctuation and line breaks stay unchanged. Their trace rows say that the keyword was not used.
Use a classical cipher for learning, not secrecy
Vigenere is historically important, but repeating-key patterns are breakable and do not provide modern confidentiality or authentication. Keyless cracking, custom alphabets and autokey variants are outside this standard known-key tool.
Local copy and export behavior
The browser performs the transformation without sending your text or keyword to a calculation service. Copy writes only the result to your clipboard. A downloaded text file always contains the exact input, result, and convention; the keyword and up to 200 trace rows are each added only when you select their separate options.
Frequently asked questions
Why didn't punctuation change?
This tool shifts ASCII A–Z letters only. Spaces, numbers, punctuation, line breaks, emoji and non-ASCII characters are copied to the same position so the message layout stays intact.
Does punctuation advance the keyword?
No. Only an ASCII A–Z letter consumes the next keyword letter. The letter-by-letter steps mark every preserved character as ‘keyword not used,’ so you can verify the alignment.
Why didn't é change?
Accented letters are outside this page's explicit ASCII A–Z alphabet. They remain exactly as entered and do not advance the keyword. The tool does not normalize or transliterate your text.
What does A=0 mean?
The formulas number A through Z as 0 through 25. Encoding adds the plaintext and keyword indices modulo 26; decoding subtracts the keyword index and wraps back into the same range.
Why is the Vigenere cipher not secure?
A repeating-key classical cipher has patterns that can be analyzed, especially when messages are long or keys are reused. It is useful for historical study and puzzles, not for passwords, confidential records, or modern data protection.
How is Vigenere different from Scytale?
Vigenere shifts each A–Z letter using a repeating keyword. Scytale is a transposition cipher: it changes character positions by writing and reading a grid. They teach different classical-cipher ideas.