There are several options.
1) If you have access to a digital signature, even a self-signed digital certificate, you can digitally sign the data. On the email submit button, click Sign Submission and select Sign Data Only on the Sign Data tab.
2) If you have access to a digital signature, use the JavaScript crypto library. See https://developer.mozilla.org/en/JavaScript_crypto.
3) Use a JavaScript implementation of a hash-based message authentication code such as MD5 or SHA-1. For example, see http://jssha.sourceforge.net/.
Steve