What it does
The Vigenère cipher improves on the simple Caesar cipher by using a repeating keyword instead of a single fixed shift — each letter of the keyword determines a different shift for the corresponding letter of the message, making basic frequency analysis much harder.
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How it works
- The keyword is repeated to match the length of the message, skipping over any non-letter characters in the message (they're left untouched and don't consume a keyword letter).
- Each message letter is shifted by the position of the corresponding keyword letter in the alphabet (
A/a= shift 0,B/b= shift 1, etc.) — exactly like a Caesar shift, but the shift amount changes for every letter. - Decrypt reverses the shift using the same keyword.
- Letter case is preserved; digits, spaces, and punctuation pass through unchanged.
Like the Caesar cipher, this is a historical/educational cipher — not suitable for real security. Everything runs locally in your browser.