Various Resistive Random Access Memory (ReRAM) devices have been used to generate cryptographic keys. The physical characteristics exploited are often related to the forming of conductive filaments, as well as the programming of cells. In this paper, key generation methods based on pre-formed ReRAM cells are analyzed. An evaluation of the bit error rate (BER) of cryptographic keys is conducted by analyzing physically unclonable function arrays that have been exposed to changes such as temperature drifts, aging, and other factors. Understanding and utilizing this data for security requires an insight of the behavior of physical elements under varying temperatures and currents. In order to guarantee maximum data security by leveraging cryptographic key generation with these methods, we must ensure that keys have low error-rates, which is only possible by producing stronger keys. We are reporting experimental data showing conditions in which the bit error rates are as low as 10 - 6.