The security of MBMS provides different challenges compared to the security of services delivered over point-to-point services. In addition to the normal threat of eavesdropping, there is also the threat that it may not be assumed that valid subscribers have any interest in maintaining the privacy and confidentiality of the communications, and they may therefore conspire to circumvent the security solution (for example one subscriber may publish the decryption keys enabling non-subscribers to view broadcast content).
Countering this threat requires the decryption keys to be updated frequently in a manner that may not be predicted by subscribers while making efficient use of the radio network. The stage 1 requirements for MBMS are specified in TS 22.146
The Technical Specification covers the security procedures of the Multimedia Broadcast/Multicast Service (MBMS) for 3GPP systems (UTRAN, GERAN and E-UTRAN). MBMS is a 3GPP system network bearer service over which many
different applications could be carried. The actual method of protection may vary depending on the type of MBMS
full Table of Contents for TS 33.246 Word version: 15.0.0