Security properties of oscillator rings in true random number generators

In order to achieve high security in a true random number generator (TRNG) consisting of several equal length oscillator rings implemented in a field programmable gate array (FPGA), it is important that the rings do not oscillate with identical frequencies and phases. In such a degenerate case, the entropy of the TRNG output bit sequence would be drastically reduced. In this paper, an investigation of the properties regarding interaction between the oscillator rings, the dispersion in the ring frequencies and correlation and dependencies between oscillator rings is carried out in three different FPGA device families. The experiments show that there are interactions and correlation between oscillator rings, and these effects need to be compensated in order to improve the security of such a TRNG design.