Electromagnetic Side-Channel Analysis Against TERO-Based TRNG

True random number generators (TRNGs) based on ring oscillators are employed in many devices because they can be constructed with logic gates only. Random numbers generated by TRNGs are used for various cryptographic systems as session keys, nonces, and masks. The randomness of a TRNG is characterized by uniformity, nonreproducibility, and unpredictability. Moreover, the quality of random numbers affects cryptographic systems. In particular, degradation of the unpredictability can reduce the security of the cryptographic system because random numbers can be easily estimated. Side-channel attacks, which exploit additional information leaked from a cryptographic module to reveal secret information, are well known. If some additional information reflecting the output bit can be measured from electromagnetic (EM) emission as a side-channel leakage against a TRNG, the unpredictability of the TRNG may decrease. Accordingly, this article introduces the leakage model that reflects an output bit generated by a transition effect ring oscillator (TERO) based TRNG, and an attack that estimates random number bits by measuring EM emission from an integrated circuit against TERO-based TRNG. We propose a leakage model against a TERO-based TRNG and demonstrate that the output bits of the TRNG can be estimated by analyzing the radiated emissions. We also consider a countermeasure against the proposed attack.