Possible use: To send someone sensitive information such as your login and password.

To use, just enter your text to encrypt, plus a passkey, then click Encrypt. The encrypted data will replace your text. Send this to the recipient who must already know or be somehow told the passkey by other means.

To decode, enter the encrypted text and the passkey. Hit Decrypt, and the original message will appear.

How secure is this? It should be about as secure as you can reasonably get. The encryption algorithm is AES, which is the cipher adopted by the US government and approved by the NSA for top secret information. The javascript library I used is open source, and you can download both it and this page's code to peruse. All processing is done in your own browser, so neither the passkey nor the original message is transferred over the network at any time.