I use this tool routinely to encrypt passwords, messages, and other pieces of information that I don't need anyone to know about.
I'm sharing it here since the tool is useful, simple, and might spark some interest in computer science and cryptography. It is not a complex piece of software.
It would not be hard at all to rebuild this tool yourself. I encourage rebuilding it. It simply makes use of the CryptoJS library. It's been a while since I looked at the original source, but it appears you could use this repository to get started https://github.com/brix/crypto-js.
Many apps currently use message encryption, so this tool is not unusual. Please use it for good purposes, and of course, I'm not responsible in any way for misuse.