# Phase Calibrated Ring Oscillator PUF Design and Application
## 1 Introduction  
Physical Unclonable Functions (PUF) have been studied to authenticate integrated circuits and generate cryptographic keys [1]. As a physical component, PUFs are very hard to duplicate or predict due to the results of random uncontrollable variables in the manufacturing process. Therefore, they are applied as a source of random, but reliable data for applications such as generating unique IC identification numbers or encryption keys. A PUF can be broadly classified as a \"strong PUF\" or a \"weak PUF\". The fundamental difference between weak and strong PUFs is the number of unique challenges that the PUF can process [2]. Rather than storing secrets in non-volatile memories, PUFs can provide significantly higher physical security by generating these secrets from unique PUF responses [3]. When PUF receives a challenge, it reacts with a response, which is known as a Challenge-Response Pair (CRP). An ideal PUF-based CRP provides strong advantages in that each response gives negligible information on the responses from different challenges to the same PUF or even identical challenges on different PUFs [4].  
It is essential that given the same challenge, the PUF response should be deterministic. However, most PUF constructions cannot offer reliable responses without post-processing algorithms. Thus, they are barely used directly in industry. The high Raw Bit Error Rate (RBER) in the responses of PUFs requires strong Error Correcting Codes (ECC) to guarantee a low false negative rate in the authentication process. Nevertheless, a strong ECC requires large memory space and high calculation complexity. When the decoding demands are large enough, electronics systems with limited computation and storage capabilities may not be able to afford the burden. As a result, traditional PUFs do not offer enough reliability for industrial applications [5].  
In this paper, we focus on a practical PUF design on FPGAs. We thereby make the following contributions:We improve the accuracy of Ring Oscillator (RO) frequency measurement on FPGAs. The proposed phase calibration solution provides efficient estimation of RO frequencies.
* We improve the PUF stability with limited hardware resources. The cost is half of the traditional RO PUFs.
* We provide a variable output width of responses. Thus, our PUFs are more flexible for different applications.
* We explore the instability feature of PUF responses. A new design is provided by converting unstable RO pairs to the stable output.
* We show an obfuscation strategy by using the instability of Phase-Calibrated PUF (PCPUF) responses.
* We discuss the authentication application based on the novel PCPUFs.  
The remainder of the paper is organized as follows: Section 2 describes the related work on RO PUFs. Section 3 introduces our RO PUF design and implementation on FPGAs. Section 4 discusses the phase calibration process on our PUFs. Section 5 gives a further improved solution of stable response generation and talks about the applications of our PUF. Section 6 shows the experimental results of our PUFs. We close with conclusions in Section 7.