# New Security Improvements in Next-Generation Passive Optical Networks Stage 2 +
## 2 Related Works  
There are many types of PUFs that can be used in an ONU, for example, optical physical unclonable functions [9] and memory-based PUFs. [PERSON] [10] described an optical PUF that uses an interaction between the laser beam and the transparent optical token with an inhomogeneous microstructure to generate a speckle pattern. The disadvantage of this PUF is that it requires demanding external measurements. The output response depends on the location of the optical token, the laser orientation, and the wavelength of the laser. The authors of [11] proposed a physical unclonable function based on a single optical waveguide that they experimentally and numerically validated. The system's responses (responses of PUF) consist of speckle-like images that stem from mode-mixing and scattering events of multiple guided transverse modes [11]. This PUF again requires demanding external measurements. [PERSON] et al. [12] proposed a photonic PUF based on ultrafast nonlinear optical interactions in a chaotic silicon microcavity. The device is probed with a spectrally encoded ultrashort optical pulse that nonlinearly interacts with the microcavity [12]. This PUF is fast, simple, compact, and low cost. [PERSON] et al. developed a tamper indicating optical PUF using polyurethane adhesives with dispersed nanoparticles, where authentication is performed by the use of a wavefront-shaping controlled reflection [13]. This PUF requires demanding external measurements. There are a lot of another types of PUFs, which can be used in our security improvement. For example, the magnetoresistive random-access memory (MRAM) PUF, the dynamic random access memory (DRAM), PUF, and the memristive device based strong PUF (mrSPUF). MRAM PUF responses are generated using the unique energy-tilt, which is an outcome of the random geometric variations in the MRAM cells [14]. Their solution requires only minimal hardware and current drivers beyond conventional MRAM to supply the destabilizing current. The MRAM PUF generates a very high entropy, a low intra-distance, and a very high inter-distance. The MRAM PUF is easily constructible and evaluable and has small requirements to the area. The DRAM PUF [15; 16] relies on the capacitor in the DRAM to initialize to random values at start-up time. The DRAM PUF provides a large number of input patterns (challenges) compared with other memory-based PUF circuits such as static RAM PUFs and can be used in low-cost identification applications. The mrSPUF [17] exploits the extremely large information density available in nanocrossbar architectures and the significant resistance variations of memristors. This PUF provides large challenge-response pairs (CRP--a challenge is a input for PUF and a response is an output of PUF), desirable characteristics of strong PUFs, low-cost overhead, and can be reconfigured. There are many correction codes that can be used for the correction of PUF responses. For example, [PERSON] et al. proposed fuzzy extractors [18; 19] to generate strong keys from biometric and other noisy data. They proposed two primitives: a fuzzy extractor that reliably extracts nearly uniform randomness \(R\) (\(R\) can be used as a key in a cryptographic application) from its input (the extraction is error-tolerant in the sense that R will be the same even if the input changes, as long as it remains reasonably close to the original) and a secure sketch that produces public information about its input \(w\) that does not reveal \(w\) and yet allows exact recovery of \(w\) given another value that is close to \(w\)[19]. The authors of [20] proposed secure and robust error correction for PUFs that uses index-based syndrome coding with an encoder and decoder. The authors of [21] presented complementary index-based syndrome (C-IBS) coding, a new and flexible fuzzy embedder for PUFs. C-IBS applies IBS several times to the same group of PUF outputs. The C-IBS has low implementation complexity and can be implemented in resource-constrained devices.