The strong encryption with JavaScript implementation of David Wheeler & Roger Needham's Block TEA (Tiny Encryption Algorithm) by Chris Veness is done only on the client computer browser with JavaScript. Information is not sent or stored anywhere during encryption or decryption. So this program can be used even when the computer is disconnected from the Internet after loading this page, or the browser is in the "Work Offline" mode.
The Tiny Encryption Algorithm is the real strong encryption; there have been no known successful cryptanalysis of TEA. It is the fastest and most efficient cryptographic algorithm in existence. It's public domain. The code is lightweight and compact, what makes it suitable for viable JavaScript implementation.
The JavaScript source code of this program can be easily viewed as the source code of this HTML page. Security specialists do not recommend using the encryption programs, which source code is not available for analysis.
The password can contain letters, figures, and symbols. The longer is the password, the stronger is the encryption. The password shall not be the word from a dictionary. Such password can be "guessed" by the utility program in 1 (!) second. I would suggest the random text string from small and capital letters, figures, and keyboard symbols not less than 14 characters. Such a good password would be difficult to find out even with the supercomputer.
If the password is lost or forgotten, there is absolutely no way to recover the text from the encrypted text, as it is the strong encryption.
This encryption can not be broken in the direct meaning of this word. Still there are methods, how any encryption can be compromised. For example, the key-logging software can be installed on the computer to log every keystroke, etc. You can protect against key-logging spying by using the virtual onscreen keyboard, available in this implementation. This makes this cryptographic system practically impregnable.
The advantage of this encryption program is that it is accessible at any computer connected to the Internet. No installation is needed.