Influence of Magnetic Fields and X-Radiation on Ring Oscillators in FPGAs

Cryptographic functions are of increasing importance for all kinds of hardware devices. Their strength against attackers not only relies on the particular cryptographic algorithm but also on the quality of the underlying random number generator. Several techniques have been proposed for implementing true random number generators in digital circuits, yet their immunity against ionising radiation and strong magnetic fields has often not been evaluated. In particular FPGAs seem to be prone to such kinds of attacks, as ionising radiation and magnetic fields may not only influence logic gates but also the configuration memory. In this paper we investigate the influence of X-rays and magnetic fields on three different types of ring oscillators. We conduct experiments with a constant X-ray beam generated by a tungsten radiation source and strong static magnetic fields up to 14 T. We show that both magnetic fields and X-radiation do not have any influence on the amount of entropy generated by the ring oscillators, hence these implementations can be considered safe against such attacks. The random number generators are implemented on Altera Cyclone IV, Lattice LFE3, and Xilinx Spartan 6 FPGAs.