About this tool
The Keyword Cipher is the classic way to generate a monoalphabetic substitution alphabet from a memorable keyword. This tool builds that cipher alphabet, shows the full mapping, and uses it to encrypt or decrypt your text in the browser. It is distinct from Caesar (a fixed shift), Atbash (a fixed reversal), and Affine (a linear formula): here the entire substitution alphabet is derived from a word you choose.
The cipher alphabet is constructed by writing the keyword with duplicate letters removed, then appending the remaining unused letters of A–Z in order. An optional fill-start letter lets you begin that remaining sequence at a chosen point (wrapping around the alphabet) for extra variation. The resulting 26-letter string lines up positionally with the plain alphabet A–Z, so the letter at each index becomes its substitute. Encoding maps plain to cipher; decoding inverts the same table.
All processing happens locally, so nothing you type leaves your device. Case is preserved and any non-letter characters pass through untouched, which keeps punctuation and spacing intact. Because it is a simple substitution, the cipher preserves letter-frequency patterns and is vulnerable to frequency analysis — great for puzzles and learning, but not for real secrets. For example, the keyword KRYPTOS produces the cipher alphabet KRYPTOSABCDEFGHIJLMNQUVWXZ.