I'm going to be interested in how you propose to retrieve the data from that digest... since it returns a 16 byte one-way hash of the data. :)

Now, to give you credit, you could at least verify that the people hadn't switched up the data from form to form with this. That isn't an unimportant task and worthy of implementing even if the users decides to go with SSL only and just let HTTP and the browser resend the arguments over and over.