Build a mixed cipher alphabet from a keyword and encrypt/decrypt monoalphabetic substitution text. Keyword Substitution Cipher is a free online tool from the ToolsRift Text Encoding Tools collection. It runs entirely in your browser — nothing to install, no signup, and your data never leaves your device.
The keyword is written first with duplicate letters removed, then the remaining unused letters of the alphabet follow in order. 'KEYWORD' produces the cipher alphabet KEYWORDABCFGHIJLMNPQSTUVXZ.
It is a monoalphabetic substitution: plaintext A maps to the first cipher letter, B to the second, and so on. Each letter is always replaced by the same cipher letter, so it can be broken by frequency analysis.
Yes for letters. Encoding then decoding with the same keyword restores the original letters (output is uppercased). Spaces, digits, and punctuation pass through unchanged.