## 4 Hardware Security
### Physical Unclonable Functions (PUFs)
#### 4.1.2 Application of PUFs
have proposed to mitigate ML attacks by reducing linear correlation between the CRPs through a matrix encryption technique called Bagua matrices [240]. This technique is implemented on numerous PUF architectures, including APUF, XOR-APUF, and Multiplexer PUF (MPUF). The prediction accuracy of ML attacks almost reduces to 50% through matrix encryptions, like random guessing, and subsequently improving data security and privacy [241]. The method proposed in [242] combines PUF with Paillier homomorphic encryption or ElGamal encryption to secure data exchanges. Encrypting CRPs during transmission ensures that adversaries cannot intercept or decode sensitive information. Homomorphic encryption enables data verification without decryption, further safeguarding against attacks [242]. A CMOS-based PUF is proposed for device authentication integrated with Elliptic Curve Cryptography (ECC). Elliptic Curve Digital Signature Algorithm (ECDSA) is used in message signing, which enables devices to authenticate themselves without a need for error correction or storage of redundant data [243]. Although APUFs are strong, lightweight, and capable of generating a large number of challenge-response pairs (CRPs), they are susceptible to machine learning (ML) attacks. To counter this vulnerability, researchers in [244] have developed a protocol that authenticates both devices and servers by incorporating an APUF in the device and a PUF model on the server. A zero-transistor interface between the device and server generates \"ghost bits\" that obscure the challenge bits, making it more difficult for attackers to model the PUF accurately. Another research on cryptography methods for improving strong PUF security and functionality utilizes erasable PUFs, which delete specific challenge-response pairs (CRPs) after their usage [245]. A Configurable Dual State (CDS) PUF, featuring a Feedback Obfuscation Mechanism (FOM), is proposed to enhance hardware efficiency and defend against machine learning-based modeling attacks. The CDS PUF is configured as either a Ring Oscillator (RO) PUF or a Transient Effect Ring Oscillator (TERO) PUF based on the parity of the Hamming weight of the challenge bits. The feedback obfuscation mechanism leverages a stable count value from the RO as a dynamic mask to obscure the input challenge, effectively concealing the relationship between CRPs [246]. A Cyclic Redundancy Check (CRC) PUF alters the seed challenges and transforms the response generation by changing the CRC generator polynomial to mitigate ML-based modeling attacks [247]. A switched-capacitor PUF (SC-PUF) capable of generating stable cryptographic keys leverages metal blocks and capacitive sensing mechanisms. The proposed mechanism protects against invasive physical attacks like focused ion beam (FIB) and probing methods, with a much lower bit error rate (BER) of \(10^{-4}\)[248]. A low-cost resistor-capacitor (RC) PUF is proposed to sense voltage differences caused by the charging and discharging of RC circuits. The experimental results with RC-PUFs have shown 49% uniqueness while achieving over 98% reliability [249]. Cross-PUF attacks exploit power intake measurements from one PUF instance to compromise another, assuming both PUFs originate from the same design file and manufacturing batch. To defend against these attacks, the DRILL method, introduced in [233], integrates Dual-Rail Logic (DRL) with Random Initialization Logic (RIL). This combination reduces the signal-to-noise ratio (SNR) in the power rails and balances power consumption during the transmission of \"0\" and \"1\", making it more difficult for attackers to distinguish between the two states. A fuzzy extraction technique is proposed to authenticate biometric data within a lightweight authentication protocol that utilizes blockchains and PUFs [250]. This protocol addresses privacy and security risks, offering protection against threats such as man-in-the-middle attacks, replay attacks,and impersonation attempts.