# A Survey of Internet of Things (IoT) Authentication Schemes +
## 1 Introduction  
The number of connected devices is growing exponentially, forming the so-called Internet of Things (IoT), a large network of networks connecting smart devices such as sensors and actuators. Such devices are adopted in various domains such as public health, smart grids, smart transportation, waste management, smart homes, smart cities, agriculture, energy management, etc. [1; 2; 3].  
The requirements and limitations of the connected \"things\" raise a number of challenges, including _connectivity challenges_ for billions of devices to communicate with each other, _security challenges_ with the need to protect IoT networks from being attacked (according to a Gartner report, 20% of organizations have experienced at least one IoT attack in the last three years [4]) and at the same time from being exploited to become an attack tool (e.g., Mirai botnet [5; 6]). These challenges are \"augmented\" with the resource-limited nature of IoT devices which renders traditional communication protocols and security schemes inefficient and even infeasible for IoT. The IoT-related security issues are becoming more alarming given the ubiquitousness of IoT devices and their adoption in critical applications, which aggravate the impact of any security breach to the extent of being life-threatening. A sample scenario can be foreseen from a weakness discovered in 2017 in a pacemaker device, which led to the recall done by US Food and Drug Administration (FDA) of 500,000 pacemakers having a dread that security gaps could cause a hacker to control the heart-beat regulating device [7].  
The security requirements of an IoT network mainly depend on the type of applications it serves; the need for confidentiality, integrity and/or authentication directly depends on the security needs of the application. In particular, authentication is considered as a key requirement for IoT [3]; trusting the devices participating in an IoT network is crucial for the well-functioning of the network. A single compromised node can be turned malicious and bring down the whole system or cause disasters [1].  
The specific nature of IoT devices makes the traditional authentication schemes infeasible and not applicable. Indeed, cryptographic schemes designed for main-powered, high processing and/or large memory devices do not \"suit\" the resource-limited IoT nodes. This led to the emergence of lightweight authentication schemes; some of them are specific to the context of IoT or to the Wireless Sensor Network (WSN) (which can be considered suitable for IoT).  
This paper presents a general overview of the security concerns and requirements in the IoT environment in a layer-based approach. Then, it provides an up-to-date survey of the different IoT authentication schemes. Through a multi-criteria classification, it compares and analyzes the existing authentication protocols, showing their advantages and disadvantages, which is an extension of a previously published research work [1, 2].  
The rest of this paper is organized as follow: The generic architecture of IoT is presented in Section 2. Section 3 discusses the main security concerns in IoT and the security challenges at each layer of the IoT architecture. Section 4 provides a taxonomy of the existing authentication schemes, while Section 5 analyzes the most-known IoT authentication schemes in light of the proposed taxonomy. Section 6 provides an overview of the related works. Finally, Section 7 concludes the paper and discusses the findings of the survey.