# Enabling Trustworthy Multicast Wireless Services through D2D Communications in 5G Networks
## 1 Introduction
Even though the good results of the protocol D2D-SF have been widely demonstrated, security in communications is not taken into account. The need to look at aspects related to security stems from the fact that the 5G system is designed with security requirements in mind. In particular, the following properties make 5G a trustworthy multi-service platform: resilience, communication security, identity management, privacy and security assurance [6]. In addition to the magnified risk of security threats due to the huge number of 5G connected devices, D2D communications cause further problems due to connections happening directly between devices in proximity [7]. Several works in the literature deal with security in D2D communications. A classification and discussion of solutions to secure D2D communication can be found in [8; 9], wherein a comparison of the different approaches is also performed in terms of their ability to satisfy D2D security requirements, such as confidentiality and integrity, authentication, privacy, non-repudiation, and so on. Among these compared approaches, the work in [10] is identified as the more effective, since it is able to satisfy most of the security requirements posed by future 5G networks [8; 9]. The purpose of this work is to present an algorithm that improves the CMS performance by exploiting secure D2D communications. The proposed enhanced CMS with secure D2D communications (eCMS-sD2D) can efficiently and securely deliver multicast traffic in 5G networks. As in the cited [5], D2D clusters are formed in order to forward data towards users with the worst channel conditions, which are excluded from a first multicast transmission by the eNodeB (eNB). In this paper, we enhance the work in [5] by selecting the relay node (i.e., the RN, which is in charge to forward data sent by the eNB towards users unable to directly receive the multicast transmission) in each D2D cluster on the basis of its trustworthiness measured, by means of security mechanisms, as their capability of reliably acting as data forwarders. Thus, our proposed scheme can be seen as a clear security enhancement with respect to [5] in that it takes into account, in the D2D communications'procedure, security aspects previously not considered at all. In particular, security procedures inspired from [10] are implemented in order to guarantee confidentiality and integrity of data transmitted in D2D communications, and D2D users' privacy protection. Security mechanisms as encryption, keyed-hash message authentication code (HMAC), and signature are used to manage the message exchange between the peers. Data encryption is realized through a symmetric encryption algorithm, with the same private key used to encrypt and decrypt data. The private key is generated through an enhanced version of the Diffie-Hellman key exchange (DHKE) protocol, in which the public key exchange is intermediated by the eNodeB, representing the trusted third party. In addition, message authentication helps to avoid the man-in-the-middle attack, which represents the primary vulnerability of the DHKE algorithm. Thanks to the designed security mechanisms, many typical D2D attacks can be avoided. Among these, eavesdropping, impersonation and masquerading, besides the already cited man-in-the-middle. The detection of any security attack reduces the reliability of the node and, consequently, the probability of being selected as data forwarder. The remainder of the paper is organized as follows. Sections 2 and 3 present, respectively, the related works in this field and the basics of the proposed eCMS-sD2D. Section 4 describes the designed algorithm in details. Results from our simulative analysis are shown in Section 5. Conclusive remarks are given in the last section.