Downloadable conditional access system and method of session control for secured 2-way communication between authentication server and host device in the downloadable conditional access system

PURPOSE: A downloadable conditional access system and a method of session control for encoded 2-way communication between an authentication server and a host device in a downloadable conditional access system are provided to interexchange a message essential between a terminal and an authentication server, thereby accomplishing cross certification and encoded communication. CONSTITUTION: A key request unit(202) transmits a key request message to a authentication server. As a respond about a transmitted key message, a authentication request unit(203) transmits a authenticate request to the authentication server based on a key request message. Based on authentication request message which is received corresponding to the authentication request, a session setting unit(204) sets session with the authentication server.