## 3 Overview of Physical Unclonable Functions
### PUF Types  
The following are the various types of PUFs as shown in Figure 7. Broadly speaking, PUFs can be classified into two categories, i.e., all-electronic and hybrid. Due to uniqueness causing randomness to arise, both types of PUFs can be further classified into implicit and explicit categories. If the variations are produced because of an external force/action, then we term such PUFs as explicit, and if the variations are created internally, then PUFs can be classified as implicit. Explicit hybrid PUFs can be optical or RF-based, and implicit hybrid PUFs can be optical or magnetic-based. Explicit, all-electronic PUFs can be based on direct characterization or non-volatile memory, whereas implicit all-electronic PUFs can be classified as a racetrack, volatile, transient, or direct characterization type. Figure 7: Types of PUFs [38]. Table 4 comprehensively lists the types of PUFs, their brief descriptions, and their limitations. From the table, we can see that different types of PUFs work on different principles. Based on the strength and uniqueness of the principle, different levels of security are provided. Some PUFs are easy to hack, while some PUFs generate a response that is difficult to replicate. The details for each PUF can be found. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Study** & **Year** & **Type of PUF** & **Description** & **Limitations** \\ \hline
[39] & 2013 & Memristor PUF & Memristors are potential future-generation memory technologies. & Process variation. \\ \hline
[40] & 2013 & PCKGEN & A phase change memory-based cryptographic key generator (PCKGEN) is a technology used to create updated cryptographic keys whenever it is desired to enhance the security of a system. & Highly hackable. \\ \hline
[41] & 2014 & STT-MRAM PUF & Spin-transfer torque magnetic random-access memory (STT-MRAM)-based PUF that utilizes orientation of a magnetic layer to generate unique patterns. & Read disturb errors in MLC NAND flash memory. \\ \hline
[42] & 2004 & Acoustical PUF & In an acoustical PUF (APUF), delay lines are applied to estimate the feasibility of creating an APUF. & Highly hackable. \\ \hline
[43] & 2017 & NEMS PUF & A PUF utilizing a nano-electromechanical (NEM) switch is termed as a NEMS PUF. A particular property of the NEM-switch-based PUF is its application of stiction. & Highly hackable. \\ \hline
[44] & 2007 & RF-DNA PUF & RF-DNA PUF with an analog or digital backend. Every individual antenna can act as a transmitter or receiver of RF waves in a particular frequency band backed by the backend process. & Special requirements. \\ \hline
[45] & 2009 & LC PUF & Such PUFs are based on resonance peaks in the frequency response of inductor-capacitor circuits. & Increased storage cost for noise issues. \\ \hline
[46] & 2002 & Optical PUF & The process of light scattering by particles is unrestrained, and the contact between the laser and the particles is quite complex. This attribute can be used for creating unique patterns that can enhance the security of the system. & Building complex physical structures for authentication. \\ \hline
[47] & 2008 & Phosphor PUF & Phosphor PUFs are made up of phosphorescent particles of arbitrary shape and dimensions that are blended randomly. & Vulnerable to invasive human factors and environmental factors, including scratching, temperature, and humidity. \\ \hline
[48] & 2014 & Nanowire Distro PUF & Fingerprint patterns can be physically validated in an easy and straightforward way by using an optical microscope. & Weak for spoofing attack. \\ \hline
[49] & 2017 & Liquid Crystal PUF & A liquid crystal PUF uses \(\upmu\)m scale diameter shells of cholesteric liquid crystals (CLCs) at the surface of the physical object to be verified. & The challenge of the PUF is the number or position of the liquid crystal in the array, and the response is the frequency of reflected light. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The relationship between type of PUFs, benefits, and limitations.