Mono-Alphabetic Substitution Helper (MASH)

You can bookmark this page. Or save this HTML file locally and double-click it to load it into your browser. Or if you're on a mobile device, you can save the URL of this page as an icon on your home screen and it should work as an offline app. Or just save this link as a bookmarklet in your browser, and click the bookmark while on any page to have the helper open up floating on that page (yeah, it's by far the coolest option, but I made the rest of this page for people who don't trust bookmarklets).

To use: paste the text you want to encrypt or decrypt into the textarea at the top.
In the textfield at the bottom, replace the letters in the alphabet with other letters.
That's it, really. If you don't have 26 letters in the textfield at the bottom, it will probably transpose weirdly.

Future possibilities:
- Could use a dictionary to spot obvious words. - Hints or guesses using Furigana ("ruby text") over the raw characters. - Use bold for guessed characters, instead of upper/lowercase. - Use a separate div for the decoded text, rather than reusing the textarea. - Handle different alphabet lengths more gracefully. - I dunno, you tell me what you'd find useful!