###### Abstract  
The wide variety of services and applications that shall be supported by future wireless systems will lead to a high amount of sensitive data exchanged via radio, thus introducing a significant challenge for security. Moreover, in new networking paradigms, such as the Internet of Things, traditional methods of security may be difficult to implement due to the radical change of requirements and constraints. In such contexts, physical layer security is a promising additional means to realize communication security with low complexity. In particular, this paper focuses on node authentication and spoofing detection in an actual wireless sensor network (WSN), where multiple nodes communicate with a sink node. Nodes are in fixed positions, but the communication channels varies due to the scatterers' movement. In the proposed security framework, the sink node is able to perform a continuous authentication of nodes during communication based on wireless fingerprinting. In particular, a machine learning approach is used for authorized nodes classification by means of the identification of specific attributes of their wireless channel. Then classification results are compared with the node ID in order to detect if the message has been generated by a node other than its claimed source. Finally, in order to increase the spoofing detection performance in small networks, the use of low-complexity sentinel nodes is proposed here. Results show the good performance of the proposed method that is suitable for actual implementation in a WSN. physical layer security; wireless fingerprinting; machine learning; intrusion detection +
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radical change of requirements and constraints for establishing secure communication. In particular, cryptographic techniques can lead to excessive transmission overhead, communication latency and power consumption, which are not suitable for IoT devices with limited resources. The application of IoT services and systems spans every aspect of our lives: health, education, industry, smart homes and cities, transportation, and utilities, just to name a few. The potential benefits offered by IoT are endless and will become more effective with the increase in the number of connected devices, but this requires to address the new security challenges and threats that arise. This is particularly critical for some applications, such as e-health or remotely controlled cars [4]. IoT systems will be made of a huge number of devices that very often will have limited computation, memory and energy resources. Hence, the use of complex asymmetric cryptography schemes will be impractical in many cases. Symmetric cryptography is more suitable for many IoT devices from a complexity and energy consumption point of view, but in this case, the distribution of the keys remains a challenge. In this context, physical (PHY) layer security (PLS) is a promising additional means to realize communication security with low complexity [3] since it substantially operates independently on the higher layers and can be used to enhance the security of existing approaches. The basic idea is exploiting the randomness of the propagation channel, noise and interference to limit the information that can be wiretapped by an unauthorized user. In addition, PLS can be used to generate secure keys and to identify unauthorized users [5]. Indeed, PLS can be realized in different ways:  
* _Secret communications without encryption_--with a suitable design of the transmitted waveform (coding, modulation, precoding schemes, etc.) together with the exploitation of the available channel state information, it is possible for the intended receiver to be enabled to successfully decode the data while the potential eavesdropper is not. * _Secure key generation_--when the use of encryption is preferred, the randomness of the channel between two nodes can be exploited to generate keys to be used for symmetric encryption.