Device-to-Device (D2D) communication is an important feature for many kinds of mobile networks and in particular for the UMTS Long Term Evolution Advanced (LTE-A) cellular technology, where an underlay network can be created between User Equipments (UEs) without the support of an evolved Node Bs (eNBs). The possibility to allow direct links between UEs plays an important role when the commercial LTE infrastructure is subject to failures or becomes unavailable after a disaster. Project ABSOLUTE, in scenarios for public safety, proposes raising in the sky a balloon with attached a 4G eNB (AeNB) to restore temporarily connectivity. In all cases in which UEs are out-ofcoverage of the AeNB like in indoors, D2D becomes crucial. In this work we propose to establish direct links employing a communication protocol that relies on the transmission of beacon frames sent in broadcast mode by selected UEs. We delve security aspects of this D2D protocol suitable for Public Safety (PS) users with out-of-coverage UEs based on sharing encryption keys. We pursue a network connectivity analysis of the secure protocol whereby we are able to show the existence of tradeoff points between connectivity and the increased overhead added by the security for different values of the system parameters. ; JRC.G.6-Digital Citizen Security