This works nicely for cases where your users have accounts where their passwords are stored on your web server and you would like them to be able to pass somewhat sensitive information over the web while reducing the risk of message interception. This is actually more of a toy than a serious encryption effort but it is fun!
How it works
You or your users enter their password (encryption key).
Then a secret message or string of characters is entered.
The Javascript takes the password and uses it to encrypt the secret message.
Finally, Javascript is used to decipher the message using the key again.
Scenario One - Web Server
In the real world you don't want to submit the user's key with the secret message
Instead, your javascript should first delete the information in the password field before submitting the information back to the server.
Once the secret message is sent to the web server you can decrypt it by referencing the user's credentials and their password which is stored in a database.
You'll need to copy the script on this page and download SimpleEncryption.js for the cipher.
If you follow the algorithm in this file it's also very easy to recreate in other languages like VB Script for ASP.
Scenario Two - Email Exchanges
You and a friend may agree on passwords and with this page you can create messages to pass back and forth through email.
Have fun!