There are a couple of ways to do this:
- If you can modify the form, specify a different action target to a host/cgi that you own to log the submitted data.
- If the form doesn't have the protocol/host specified in the action, you might be able to get away with a simple ssl proxy, using stunnel or the like.
- You might be able to perform some DNS trickery after the getting the form, but before submitting it and direct the app to a web server you own with a CGI script that logs all the input variables.
- If you can't do any of the above and the app you are using uses the standard win32 networking/crypto APIs, you can hook the SSL calls in wininet. There are products available to do that such as this one.
I only suggest the crypto hooking last since it costs money, the rest of the options cost time.