Provided are a Downloadable Conditional Access System (DCAS), and a channel setting method and a message format for a 2-way communication between a terminal and an authentication server in the DCAS. The DCAS may include: a verification unit to verify an electronic signature and an integrity with respect to a message received from the authentication server; an extraction unit to extract network access information of the authentication server from the message in which the electronic signature and the integrity are verified; and a channel setting unit to set a communication channel with the authentication server based on the extracted network access information.