# Key Recovery for Content Protection Using Ternary PUFs Designed with Pre-Formed ReRAM
## 5 Implementation with SRAM and ReRAM Devices
### Comparative Analysis of SRAM versus ReRAM Schemes
This allows for an optimization of the circuitry. In this study, we found that 10 ms are enough to read 256-bit long streams. Further reductions in latencies have a negative impact on the BERs, as the measurement becomes noisy. * Crypto-analysis: One possible attack, which is a major problem for certain applications, is when the terminal device is under the control of the opponent for even a short period of time. In this instance, it is possible to read the SRAM in a matter of seconds after power off/on cycles. The bulk of the information needed for key generation can be recovered after 100 cycles, which takes about 5 min. Pairs of ReRAM cells are more difficult to attack. The two 4 Kb ReRAM arrays are tested separately, upfront, during quick enrollment cycles. The circuitry for the ReRAM PUFs is such that when the two arrays are mounted on the custom board, the user only has access to differential reads, without having access to the individual devices. Therefore, a crypto-analysis requires that 128 million pairs be read, which takes about 4.4 h. As shown in Figure 13, two 512 Kbit arrays take 9 years to be differentially read. Pre-formed ReRAMs have additional physical properties that also enhance tamper resistance:
* Ability to sense attacks: The design of sensing elements inserted in the ReRAM arrays operating in the pre-forming range has been reported [15]. An opponent exploring the ReRAM arrays without knowledge of the vulnerable cell population has a high probability of damaging these cells. The cryptosystems developed in this study avoid this population; therefore, it is possible to monitor the potential infiltration of a crypto-analyst and to detect an attack. * Self-destruct mode at low power: ReRAMs are designed to operate in the set/reset mode after the forming operation. The forming operation in a ReRAM is a non-reversible process that usually starts with voltage stress in the 1.5-volt range. In case of an attack, the user can trigger a self-destruct mode of the ReRAM cells by initiating the forming cycles. Only partial cycles are needed, as the objective is to form enough cells to make the PUF useless, for example, half of the cell population. * Radiation hardness: SRAMs are vulnerable to ionizing radiation; however, in this particular application there is a mitigation process of performing power off/on cycles before each response generation. The likelihood of several cells being impacted by radiation just before response generation cycles is small; therefore, the impact on the BER is anticipated to be limited. The ReRAM technology is known for being rad-hard [44]. The 4 Kb arrays in this study were manufactured with the conductive bridge RAM (CBRAM) technology that has been tested as more stable under ionizing radiation than the more traditional ReRAM technology that can be impacted by migrations of oxygen vacancies. The SRAM technology for the design of outstanding PUF-based cryptosystems has been demonstrated by industrial suppliers and academic institutions. It is not the objective of this analysis to criticize SRAMs in favor of largely unproven technology. The authors acknowledge that it will take years of research before ReRAM technology reaches similar levels.