Communication protocol for secure communications systems

A method and apparatus for authenticating a key management message within a secure communication system is provided herein. During operation, a digital signature for message authentication of a Project 25 Key Management Message (KMM) is utilized. In particular, the digital signature will be used to authenticate the KMM in scenarios where there is no Message Authentication Code (MAC). The MAC will be utilized to authenticate the KMM when available. Because authentication of KMMs take place, even when no MAC is available, it becomes increasingly more difficult to tamper or spoof the delivery of encryption keys.