ILLEST on your computer.
Use ILLEST in your browser with nothing to install, or as a desktop app for Windows that carries its own code. One account works in both.
ILLEST, installed.
The same ILLEST in a window of its own, with its code built in.
Its code is its own
In a browser, the page's code comes from the server each time, so whoever runs the server could change it. The desktop app carries its code inside the app.
Your key, in the keychain
Your device's key is kept in your system keychain and used by the app's own process. What that does not cover is set out under Messages.
For Windows
The installer sets the app up for your own Windows account.
Or open it in your browser.
ILLEST opens in your browser. Free to use, and nothing to install.
In a browser, the page's code comes from the server, so whoever runs the server could change that code to capture your password. The desktop app carries its code inside the app instead of fetching it from the server each time, so it does not have that weakness.
Come in.
ILLEST opens in your browser. The desktop app is the other way in; what it protects, and what it does not, is set out under Messages. Free to use. Bring your people.