# New Security Improvements in Next-Generation Passive Optical Networks Stage 2 +
## 5 Security Analysis of Our Proposal of Security Improvements in the NG-PON2  
Our security improvement SI-NG-PON2 is based on implementing a PUF with a correction code. As the secret information Registration_ID_PUF (used for authentication and generation of the MSK and derived shared keys) is not stored in nonvolatile memory in an ONU, but is instead generated as needed by using the received challenge and helper data, an attacker cannot copy this secret information from the ONU to an another device. A challenge and helper data do not reveal any information about the generated response by the PUF and CC. For these reasons, the attacker cannot clone an ONU, cannot act as another ONU and cannot decrypt an encrypted communication. A malicious user is not able to replace or reprogram an ONU for receiving all downstream data intended for all connected users since he does not know their secret information (Registration_ID_PUF) and there is no way to calculate it without the PUF and CC implemented in the ONU. For the same reason, a malicious user is not able to replace or reprogram an ONU and forge packets (upstream data) to impersonate a different ONU. The bit-flipping attack is not possible, as we use the AES-CMAC algorithm with a 128-bit key for encryption of the XGEM payloads and data encryption keys in our proposal of security improvement. Next, we use AES-CMAC-64 for integrity protection and data origin verification of PLOAM and OMCI messages in our proposal of security improvement. The CMAC encryption mode ensures confidentiality, authenticity and integrity of binary data. If the attacker changes some transmitted data (performs a bit-flipping attack), the subsequent check will reveal this change. The guessing attack to integrity protection and data origin verification of OMCI messages is not possible since we use the AES-CMAC-64 algorithm. The 64-bits length of a key is enough to satisfy the protection against the guessing attack. If an attacker connects a malicious device at various points in the infrastructure that could intercept and/or generate traffic, the malicious device will could not impersonate an OLT CT, as the mutual authentication between an OLT CT and an ONU is mandatory in our proposal of security improvement in NG-PON2. A new connected malicious device could not impersonate an ONU since an OLT CT does not have challenge-response pairs of the connected malicious device in its CRP database. Figure 4 describes a situation when an attacker comes between an OLT CT and an ONU, and, due our security improvements cannot perform the cloning attack, bit-flipping attack, guessing attack, and impersonation attack. Our security improvement (SI-NG-PON2) is based on the using PUFs in ONUs. The main condition which must be carried out in order the using of a PUF was safely is that the intra-distance of the PUF approaches 0.00% and the inter-distance of the PUF approaches 50.00%. Equations (2) and (3) show a mathematical expression of the intra-distance and inter-distance calculation [42]. Figure 4: Unfeasible attacks on optical network unit (ONU) and optical line termination (OLT) channel termination (CT) in our security improvements. \[D_{puf_{i}}^{\text{intra}}(x)\overset{\Delta}{=}dist[Y_{i}(x);Y_{i}^{\prime}(x)], \tag{2}\]  
where \(Y_{i}(x);Y_{i}^{\prime}(x)\) are two different and random evaluations (responses) of the one same PUF instance, \(puf_{i}\), for one the same challenge x.  
\[D_{C}^{\text{inter}}(x)\overset{\Delta}{=}dist[Y(x);Y^{\prime}(x)], \tag{3}\]  
where \(C\) is a class of the PUF and \(Y(x);Y^{\prime}(x)\) are two different and random evaluations (responses) for one the same challenge x by two random and different PUF instances. We recommend a using strong PUFs in SI-NG-PON2. Since strong PUFs have a huge number of challenge-response pairs, an ONU may change Registration_ID_PUF many times during a service life of a PUF. For example, in [42] the authors presented a strong silicon PUF with \(2^{65}\) challenge-response pairs, which is immune to machine learning's attacks.