In this paper, we propose and analyze a multicast key backbone for secure group communications. When a group member joins or leaves the multicast group, the system has to update and distribute encryption keys to assure that only active members could receive the latest information. In previous tree-based multicast key management schemes, the depth of the key tree is unbounded and analytically deriving the exact value of the corresponding average update cost remains an open problem. In contrast, the depth of the proposed multicast key backbone is fixed. We show that the evolution of the multicast key backbone can be modeled as a continuous-time Markov chain or a regenerative process. We analytically derive the average update cost for a state transition. Furthermore, we use renewal theory to derive the exact value of the average update cost per time unit.