One embodiment described herein can provide a system for enabling a service over a computer network. During operation, the system receives, from a first client device, an object-distribution request. In response to the request, the system generates an object package containing a number of objects based on the request and obtains at least one challenge associated with the object package and a predetermined response corresponding to the challenge. The system then presents the challenge to a user of a second client device and receives a response to the challenge from the second client device. In response to the received response to the challenge meeting one or more of predetermined criteria, the system causes the object package to be distributed to the second client device over the computer network.