The use of delay lines in a ring-oscillator-based combined true random number generator

This paper presents a novel approach for the design of ring oscillators in a combined true random number generator (TRNG). We propose using delay elements made on carry4 primitives instead of series of inverters or latches in ring oscillators, which enables the construction of many high frequency ring oscillators with different nominal frequencies in the same field programmable gate array (FPGA). The output sequences produced by this type of TRNG pass all NIST 800-22 statistical tests for smaller numbers of ring oscillators than the constructions described in the literature. To assess the unpredictability of the output bits, the restarts mechanism, proposed in earlier papers, was used. Due to the number of ROs with different nominal frequencies and the method of construction of carry4 primitives, it is expected that the proposed TRNG is less sensitive to frequency injection attacks.