Payment Approval for PayWord

In case of micropayment schemes, all costs that appear during functioning should be minimized. This includes cost of disputes and charge backs that result in penalties for the vendor. We extend the PayWord micropayment scheme with payment approval to minimize disputes, charge backs or to avoid attacks that ruin the reputation of the vendor. Payment approval is achieved by employing a MAC function per a purchase, that does not increase time complexity significantly. A formal evaluation in applied π and a proof that our scheme fulfills secure payment authorization, payment approval and secrecy of payment information are also given.