# Total Ionizing Dose Effects on a Delay-Based Physical Unclonable Function Implemented in FPGAs
## 1 Introduction  
Securing sensitive information on low-cost satellite applications has become a major challenge for the space industry. Typical approaches that include very expensive cryptographic primitives, non-volatile memory and analogous blocks cannot be afforded in these small space systems. In this context, commercial Field Programmable Gate Arrays (FPGAs) have turned out to be a good solution due to their flexibility and cost. Among their many uses, FPGAs can be dedicated to ensuring secure satellite data.  
A popular solution to provide security in resource constrained applications, such as those using FPGAs, is on-chip Physical Unclonable Functions (PUFs). PUFs are a very promising security primitive used for authentication and key generation in IC and FPGAs. These security primitives are based on the impossibility of creating two physically exactly identical ICs due to the influence of random and uncontrollable effects during the manufacturing process. These uncontrollable influences leave measurable random marks on some features which possess the potential to generate encryption keys directly associated to a device [1]. Thus, PUFs work as an unclonable specific feature that can identify a circuit, just as a fingerprint can identify a human being. Among the various device properties that can be used for this purpose, delay-based PUFs deserve special attention due to their straightforward implementation. A delay-based PUF exploits the delay dependency on the random process variations [2]. Well-known examples of this type of PUF are the Arbiter PUF [3] and the Ring Oscillator PUF [2].  
PUFs can be used to solve an important issue related to the generation of secure encryption keys in satellite communications, removing the necessity for key storage. Nonetheless, as PUF responsedepends on some circuit features that may be affected by the operational conditions; it is important to assert the suitability of these primitives in harsh environments subjected to ionizing radiation. Ionizing radiation induces charges in the semiconductor material that can be trapped in the oxide, altering the electrical characteristics of electronic devices. This effect, known as Total Ionizing Dose (TID), is cumulative and produces a gradual degradation of major electrical parameters, such as threshold voltage and leakage current, that can eventually result in device failure at a certain dose. To the best of our knowledge, the effects of TID in delay-based PUFs have not been studied before. There is one work that studies the TID effects in a CMOS silicon PUF based on transistor breakdown [4].  
In this work, we evaluate the effects of ionizing radiation on a well-known delay-based PUF [2] implemented in a SRAM-FPGA. To that end, we have performed an extensive test with two different devices exposed to a radiation source, periodically collecting the PUF response as TID increased. All the external influences that can affect the PUF response (temperature, humidity, voltage, etc.) were controlled. Several major quality metrics have been used in order to assess the impact of radiation in the PUF response.  
The remainder of this paper is organized as follows. Section 2 introduces the RO-PUF under study and the typical effects of ionizing radiation on SRAM-FPGAs. Section 3 describes the implementation of the RO-PUF and the TID test setup. Section 4 reports the impact of the ionizing dose on the RO-PUF. Several metrics are presented and analyzed in this section. Finally, Section 5 summarizes the conclusions of this work.