  To date, group-oriented communications have been mainly exploited for
delivering multimedia services in human-oriented communications while, in
future fifth generation (5G) cellular networks, objects will be the main
target. Internet of Things (IoT) will undoubtedly play a key role in 5G
networks, wherein massive machine-type communications (mMTC) feature a use case
as crucial as challenging since cellular IoT connections are predicted to grow
heavily in the next future. To boost capacity and energy efficiency, the 5G
network can leverage device-to-device (D2D) communications which are recognized
as an effective offloading technique. This is achieved thanks to the fact that,
in legacy D2D communications, data are directly sent from one device to
another, avoiding the crossing of the network. Obviously, the distributed
nature of such a communication paradigm and the inherent broadcast nature of
the wireless channel make it necessary to think how to secure the so called
"sidelink" transmissions. This work proposes a protocol for the efficient and
reliable management of multicast services in a 5G-oriented IoT scenario, in
which security is a crucial requirement to be met. The proposed protocol is
tailored to Narrowband IoT (NB-IoT) and makes use of D2D communications with
the aim of improving network efficiency and optimizing network resource
utilization. In addition, cyber security and social trustworthiness mechanisms
are exploited to secure D2D communications.
