With the progress of the mobile communications and computer technology, Internet becomes extremely popular and offers various services to users. People thus can use these ubiquitous public networks to perform a transaction, access a variety of network resources and transmit sensitive data conveniently. However, these transactions always raise security issues; due to the problem of transaction parties cannot authenticate each other face to face. The traditional “Cash on Delivery” is a difficult aim in network transaction. Therefore, the study of the secure mechanisms is very important for billing and web services in Ecommerce.