# New Security Improvements in Next-Generation Passive Optical Networks Stage 2 +
## 3 Security in NG-PON2
### Physical Unclonable Functions  
Physical unclonable functions (PUFs) are functions with an intrinsic random nature that use the heterogeneities and differences of physical components of a device to generate unpredictable unique responses. This response is called a fingerprint of the device. Heterogeneities and differences of physical components of the device are random, and they cannot be controlled by a manufacturing process. For this reason, PUFs cannot be cloned. PUFs represent an alternative to the classic storing of secret keys, which are stored in nonvolatile memories. In PUFs, a unique bit string is generated for a device when needed without the need to store it. The main properties of PUFs are that they are constructible, evaluable, reproducible, unique, identifiable, physically unclonable, truly unclonable, mathematically unclonable, unpredictable, tamper evident, and one-way [33]. The first proposals to meet some properties of PUFs were defined 30 years ago; however, their authors did not call them as PUFs at that time. [PERSON] was the first to have systematically written about the concept of PUFs in his dissertation thesis [10], in 2001. [PERSON] called PUFs physical one-way functions (POWFs) in his thesis. An input of a PUF is called a challenge and an output of a PUF is called a response. The definition \(Response=PUF\) (\(challenge\)) describes a calculation of the PUF response. Assigning the response to the challenge is called a mapping process that results in a CRP. CRPs are stored in the secure database. PUFs represent a certain type of true random number generator. One challenge of a PUF represents a query for a random number. The response of PUF is the random number created by a stochastic physical process. PUFs can be used for authentication (biometric authentication schemes work similarly), generation of encryption keys, and solving an unauthorized increased production by a third party (the night-shift problem) [34]. If produced chips have implemented PUFs, the CRPs of unauthorized produced chips will not be in the database of CRPs; moreover, if they are used, authentication will be unsuccessful. PUFs can be also used as a signature scheme. A challenge represents a string of a message and a response represents a signature of a message generated by an internal private key that cannot be programmed through a manufacturing approach. The advantages of PUFs include a price reduction (it is not necessary safely store the Registration_ID i a nonvolatile memory in an ONU) and an increase of security (PUFs are resistant to reverse engineering). PUFs do not require a permanent repository to store an authentication or encryption key. This key can be generated when needed on the device at any time. The main disadvantage of PUFs is noise in output responses. PUFs generally generate noisy responses, and they contain a limited amount of entropy. For this reason, the generated response of a PUF cannot be used as an encryption key directly. The encryption key is constructed from the generated response using helper data. Errors in output responses of PUFs may have a random or deterministic nature. Random errors are caused by the noise of a peripheral device. Deterministic errors are caused by temperature differences around the device with a PUF, the aging of components of the device, an unstable supply voltage (even small voltage changes can affect to the PUF's behavior), or anything that causes variations in the internal components of the device. Deterministic errors cannot be accurately predicted, there is no equations, which can be describe changes in PUFs behavior caused by deterministic errors. Every deterministic error has a different effect on each PUF and changes its behavior differently. Correction codes are implemented together with PUFs for a correction of noisy output responses of PUFs in order to generate encryption keys. A disadvantage of correction codes is that they need a permanent memory.