A system and method for transmitting/receiving encryption information in a mobile broadcast system supporting broadcast service (BCAST) are provided. In the mobile broadcast system, a BCAST Subscription Management (BSM) manages subscriber information of a terminal, and transmits to a BCAST Service Distribution/Adaptation (BSD/A) a first delivery message including a Registration Key Material (RKM) provided for registration of the broadcast service of the terminal and including at least one service or content's identifier. The BSD/A transmits to the BSM a first delivery confirmation message including information indicating success/fail in receipt of the first delivery message, and transmits the RKM to the terminal.