Webassist eCart can help with everything except the encryption. Look at php mcrypt library to help with encryption: mcrypt
Other than that just run the checkout wizard as normal... select any local checkout payment gateway, and use authorize.net without express checkout... since "none" is not an option... just put in fake values and remove the local checkout server behavior on the confirm page when you are done.
Then you would just want to encorporate encryption and storing the Credit card information, which isn't done by default but could be added by updating the store cart summary in database server behavior on the confirm page.
The admin section would of course need reverse encryption. You could have them enter a key or even store the key in an admin controlled php file and automatically decrypt.
