Abstract: A registration module registers a first account for the first user and a second account for a second user. A communication module provides a first interface for the first user to generate a challenge message and submit the challenge message, and a second interface for the second user at the second computer system to receive the challenge message, to generate a response message in response to the challenging message, and to submit the response message for viewing on the first interface. A billing module creates a binding between the response message and the challenge message if the challenge message is the last challenge message before the response message and the response message is the first response message after the challenge message. A compensation module transfers compensation from the first account to the second account due to the binding.