A Study on Output Bit Tampering of True Random Number Generators Using Time-Varying EM Waves

True random number generators (TRNGs) based on ring oscillators are employed in many devices because they can be constructed with logic gates only. If the randomness is degraded by the attack, it may affect the security of the cryptographic protocol. As a physical attack against TRNG, several attack methods that reduce its randomness have been proposed. On the other hand, TRNG is recommended to implement an online health test to guarantee the quality of random numbers, and if this module exists, conventional attacks would be difficult. In this paper, we have shown the possibility of tampering with a part of the random number sequence by controlling the electromagnetic waves applied to the TERO-based TRNG. Specifically, disturbance waves are applied to TRNG to change the power supply voltage of the device. By modulating the amplitude of the disturbance wave, some of the random number bits are tampered with.