A cipher method and system for multicast service. A group includes multiple user equipment, each having a respective key. The keys are orthogonal to one another. A content provider provides a raw message to the user equipment of the group. A BM-SC server has a key set composed of the respective keys of all user equipment of the group to encipher the raw message to thus obtain an enciphered message for broadcast. All user equipment of the group receives and deciphers the enciphered message with the respective keys. When a user equipment leaves the group, the key set held by BM-SC is updated by removing the key of the leaving user equipment.