Create a secret password known only to you and another party, without ever communicating it. Uses the TweetNaCl-js implementaion of Curve25519 Diffie-Hellman key exchange. Optionally, short text messages may be encrypted/decrypted with the secret password using Salsa20 and Poly1305.
Github page with usage instructions: https://shared-secret.github.io
The above HTML page, once served to your computer, has no further communication with the server or any other entity. No cookies or local data storage are created when the pages run in your browser. Therefore, secret data exist only temporarily within the browser while it is open, unless manually copied elsewhere. The browser cache should be cleared before closing it. The clipboard should also be cleared if a secret was copied to it.
For enhanced peace of mind, save the HTML page to your computer when it is blank and move it to an offline computer before using. Review the dowloaded code, which includes the two libraries nacl.min.js and nacl-util.min.js, obtained from github user dchest (Dmitry Chestnykh). The originals are available at these links along with non-minified (and possibly more recent) versions: