Influence of operating conditions on ring oscillator-based entropy sources in FPGAs

True random numbers play an important role in the implementation of cryptographic functions and protocols. In case FPGAs are used as implementation platform, true random number generators also must be realized on the FPGA. While such generators have been published, it remains unclear, whether they are safe against external manipulations. Changing the physical conditions under which the system runs, might influence the amount of entropy that can be harvested from the entropy sources. Thus, system security could be compromised if the random number generator fails or delivers random numbers of a low quality. In this paper we show that two types of true random number generators are safe against temperature and voltage attacks on at least two different types of FPGAs.