## 2 Related Works
### New Approaches to Secure M2M and IoT Communications  
Centralized architectures suffer from Single Point of Failure (SPF). As an alternative solution, new decentralized approaches have been recently proposed to secure M2M communications.  
Blockchain-based protocols are gaining popularity, and appear very promising to address the security challenges of M2M and IoT networks. Blockchain allows all transactions to be recorded in a shared database or a DL (Distributed Ledger). Hence, any modification can be easily detected. Theoretically, blockchain has the ability to provide security services as integrity, authentication and access authorization. However, there are challenges to overcome when used in M2M and IoT contexts due to its limited scalability and the constraints of the devices.  
The authors in [19] use a blockchain-based approach to secure the routing process while considering blockchain as shared memory in WSN. In order to optimize and to secure the routing phase, the blockchain preserves track of the generated transactions on the network stating which nodes are transmitting and through which paths.  
A blockchain-based information securing process, in which data is collected from the sensors utilizing an Unmanned Aerial Vehicle (UAV) as a relay, is presented in [20]. The collected information is safely stored within the blockchain on the mobile edge computing (MEC) server. Within the proposed scheme, the information is encrypted before transferred to the MEC server with the help of UAV. Upon getting the information, the MEC server approves the information and sender's identity. Effective validation is followed by storage of the information within the blockchain.  
In [21], the authors present a blockchain based scheme in combination with a proxy re-encryption mechanism to guarantee the privacy of the information. The corresponding transactions are consequently managed through the agreed smart contract, and saved within the blockchain.  
A decentralized blockchain architecture is adopted in [22] with the main purpose of facilitating secure information exchange within an APL (Adaptive Protection Platform).The architecture increases the throughput while leading to better scalability, and low node storage.  
A lightweight identity management approach is presented in [23]. It is based on consortium blockchain for the IoT. The protocol addresses privacy and security issues in traditional centralized systems and centers on the concept of dispersing the authority of an identity system to a bunch of organizations. The proof of concept (PoC) as moderately lightweight consensus model is implemented.  
A lightweight authentication scheme based on consortium blockchain is proposed in [24]. A cryptocurrency concept is used to build trust between entities. The proposed framework incorporates a PKI system to identify permissioned entities within the blockchain.  
In [25], the proposed model provides a secure observed area for checking IoT data generated from IoT devices. Using the MAM (Masked Authentication Messaging) protocol, the encrypted environmental sensor data is broadcasted, stored, and fetched from a distributed ledger for the IoT, named IOTA to ensure integrity, security, and privacy.