how do you encrypt a form field such as a credit card number?

I am trying to encrypt only one field on a form.  Is that possible?

Or, I have tried to use the password option in Text Field Properties and like the idea of having the characters hidden with an asteric however how do I:

1) remove the astirik

2) save and view when i reopen the form.  The field values seem to disappear.

Please advise.

Thanks!

Dan