After receiving the randomly generated public key MN_Pub from the master node, the CA server generates a random message CA_Rand and specifies the life cycle for the public key MN_Pub of the master node. Specifying the life cycle of MN_Pub also regulates the life cycle of the corresponding user session that will be generated in the next phase. The CA server signs then a proxy credential for the current request of the master node by using the technique of Digital Signature.