Encrypt form field on submission prior to saving

I have a form that asks for a social security number. We don't want to save the SSN in plain text. I thought I could modify the Generated Code for the form. But I can't find it. It says it's located in @~/App_Code/CMSClasses@ but that path doesn't exist on the server. I tried copying the Generated Code and adding to the App_Code Folder with my modifications to Encrypt the field. But my modified code was never hit by the form submission. Is there any way to do this without making the form a custom widget?