# New Security Improvements in Next-Generation Passive Optical Networks Stage 2 +
## 3 Security in NG-PON2
### Physical Unclonable Functions
If PUFs are used for authentication of a device, an additional implementation of an appropriate correction code for correction output responses of PUFs will not be required. It uses the fact that generated responses of PUFs are sufficiently different from each other. Therefore, even if the output response of a PUF contains a greater number of errors, the device can be correctly identified. The calculation of the Hamming distance is usually used to make a decision regarding whether the received response is equal to the stored response in the database. The initialization phase is a process whereby challenges are inserted many times on the input of the PUF, and the generated output responses together with the helper data are mapped to CRPs and stored in a database. This phase must be carried out in a secure environment (by a secure channel, where an attacker cannot eavesdrop the communication between the OLT CT and the ONU), usually after the production of a PUF. CRPs are used in the authentication phase, and CRPs and helper data are used in the key generation phase. A verifier that has a database with CRPs sends a challenge to a device in the authentication phase. The device calculates a response from the received challenge by its PUF and sends the response to the verifier. The verifier compares the received response with its response that is stored in its database. If the Hamming distance between the received response and the stored response in the database is sufficiently small, the device will be authenticated. If the Hamming distance between the received response and stored response in the database is too large, the device will not be authenticated. A positive decision about authentication depends on the place where the intra-distance histogram and inter-distance histogram overlap. The intra-distance histogram represents the distance between the two responses that were made by one PUF by using the same single challenge (the required value of the intra-distance is equal to 0%). The inter-distance histogram represents the distance between the two responses that were made by two PUFs by using the same single challenge (The required value of the inter-distance is equal to 50%). The threshold for optimal authentication (equal error rate (EER)) is somewhere in the gap between these histograms; see Figure 1. If these histograms overlap, the device can be incorrectly identified (false acceptance rate (FAR)) or cannot be identified at all (false rejection rate (FRR)). To achieve the appropriate decision level, it is recommended to minimize the sum of the FRR and FAR. An example of an authenticated device can be a smart card (a radio frequency identification (RFID) tag), and an example of a verifier can be a smart card reader. An entity that has a database with CRPs and helper data sends a challenge of corresponding helper data to a device in the key generation phase. The device calculates a shared key from the received challenge and helper data by its PUF and correction code. The shared key may be used for authentication of the device or encryption of data.