Type your text and a keyword, and the tool builds a mixed cipher alphabet from that keyword and substitutes every letter through it.
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.