## 1 Introduction  
Wireless sensor networks (WSNs) are intensely ubiquitous and encompass a broad range of new applications, such as AR/VR, Internet of Things (IoT), and vehicle network [1]. The smart sensors are the tentacles that sense the environmental conditions and communicate the information that is usually security and private critical. It is paramount to assure the integrity of the sensing and protect the collected information from malicious attacks during the data transmission through the untrusted communication channels. However, it is not feasible to implement the conventional cryptography modules on the resource-constrained sensor nodes, including advanced encryption standard (AES), elliptic curve cryptography (ECC) and so on. In light of this, the physical unclonablefunction (PUF) as a lightweight security primitive has raised more and more interests in the WSN research community [2, 3].  
PUF, broadly categorized as \"weak PUF\" and \"strong PUF\", is an emerging low-cost on-chip secure primitive [2, 4, 5, 6, 7]. By converting the inherent uncontrollable and unpredictable variations of the semiconductor manufacturing process to a random digital bit string, a PUF generates a \"response\" upon a \"challenge\", namely a challenge-response pair (CRP). Weak PUFs target the application of the key generation [2, 4], while strong PUFs are more suitable for the device authentication [8]. The weak PUFs generally have very few CRPs. They can be essentially regarded as a special form of memory, which are resilient to the invasive attacks [8]. The strong PUF, as a disorderal physical system, features complex challenge-response behavior and large challenge-response space. It is almost impossible to physically clone a strong PUF, whose CRPs behave exactly the same as the original one. As a result, the strong PUF is more suitable for the wide range of device authentication applications than its counterpart of weak PUF [9, 10, 11]. Among the strong PUF implementations, ring oscillator (RO) PUF is superior due to the following reasons [12]: (1) the frequencies of the ROs in the PUF are irrelevant to the delay introduced by the outputs' routing; (2) the difference of the RO pairs' frequencies can be further increased by extending their oscillation time. However, the feature of public CRPs' accessability for strong PUFs renders them vulnerable to modeling attacks [8]. In addition, [PERSON]'s work demonstrates that the electromagnetic (EM) measurements are capable of disclosing both the frequency and the location of each RO, which enables the prediction of the RO PUF's CRPs [12].  
To address these security threats, this paper presents the first RO based strong PUF design to resist the EM side-channel attack and the support vector machine (SVM) modelling attack. Compared with the previous implementations, the proposed RO PUF features low power consumption, high area efficiency and improved security performance with high entropy. Specifically, in order to minimize the influence of the emitted EM signal, the RO's delay cell consists of the current starved inverters, which operate at the subthreshold region and significantly reduce the overall power consumption. In addition, each RO used to generate the CRP includes one of the two current starved inverters in each inverter stage, resulting in the exponential increment of the RO number in a given area. Furthermore, the RO PUF architecture employs a linear feedback shift register (LFSR) counter to enhance the system's logical reconfigurability and thwart the SVM modelling attack. The rest of the paper is organized as follows. Section 2 introduces the architecture and operation of proposed RO PUF. Experiments results are presented and discussed in Section 3. Finally, the conclusion is drawn in Section 4.