A method and apparatus for transmitting/receiving encryption information in a mobile broadcast system providing broadcast service (BCAST) are disclosed. In the mobile broadcast system, a broadcast (BCAST) Service Distribution/Adaptation (BSD/A) unit comprises a transmitter for transmitting a Registration Key Material (RKM) request message for requesting delivery of an RKM for registration of the BCAST service of a terminal to a BCAST Subscription Management (BSM) unit managing subscriber information of the terminal, and a receiver for receiving an RKM request response message from the BSM unit. The RKM request message includes an identifier (ID) of the BCAST service, and the RKM request response message includes the ID of the BCAST service and the RKM. The transmitter transmits a Long-Term Key Message (LTKM) request message for requesting delivery of an LTKM provided to the terminal during subscription of the BCAST service to the BSM unit.