## 7 Challenges and Future Directions
### Lightweight Authentication Frameworks  
One of the major elements of IoT is security and privacy. It is becoming a major challenge to ensure the privacy of user data. If an attacker can enter the system, s/he can find out the way of doing DoS or MITM attacks by getting a clear idea of the communication framework. The level of vulnerabilities will be increased further if the system is in an open environment rather than a close environment. This article gave a brief overview of existing authentication frameworks for different IoT applications. RSA based authentication scheme needs more key size length compared to ECC based mechanism. Feasibility checking of ABE and ECG based methods need to be carefully designed for the IoMT network. Attributes will be changed from time to time and it will affect the authentication. Like ABE, ECG parameters may fluctuate based on the health condition. By physically accessing the device, an attacker can get the encryption key which can be used to get data of future communications. On the other hand, direct uses of the PUF key can be subjected to modeling attack. Many PUF based authentication methods require storing challenge which is contradictory to PUF benefits (removal of storage). Moreover, PUF based methods also require a separate module or logic gates to be embedded in the device which may raise complexity or increment the size of the device. There might will some impact on prices as well. Furthermore, to avoid server impersonation attack, researchers used PUF in cloudserver as well. As server can receive multiple requests at a time, how the server will process the request using PUF is not clarified. Also, PUF increases the database size of the cloud server and for a large network, server needs to have additional resources to make query to the database. Different frameworks use many authentication steps and complex calculations to complete the authentication process, which increases computation cost and communication overhead. Authentication protocol needs to be designed with fewer numbers and sizes of messages with simple calculations. Instead of using static structure, dynamic structure can provide same level of security with less data exchange. FL can be a good option for authentication, which uses a local model to the update global model by cooperative training and getting data. Also, the power of AI can bring an advantage to complete the authentication process. Most of the researchers focused on vehicle to server secure communication. It is required to put focus on the vehicle to others' communication to make a secure and complete IoV environment. Mobility is also required to be considered for IoV or other applications as vehicles/devices might move outside of the current serving region. While considering authentication framework, cloud security needs to be ensured for centralized methods, which is a major research area. Cloud could be public or private. Cloud storage is vulnerable to data manipulation and illegal resources, which could interfere with the process of smart networks. Decentralized authentication can be applied for both homogeneous and heterogeneous networks. Elements of blockchain network such as smart contract need to be robust enough to maintain security. With the introduction of 6G, more devices with large data will be involved in the network. To ensure the authenticity and reliability of information, data anonymity needs to be addressed in the form of cipher, hash, random nonces, masking, PUF, etc.