Exploring active manipulation attacks on the TERO random number generator

True random number generators (TRNGs) are critical components in security systems used to generate session keys, challenges for authentication protocols and masks for secret sharing. Unfortunately, TRNGs are vulnerable to a wide class of physical attacks ranging from passive observation of generated numbers to active manipulation. In this work we investigate the susceptibility of the Transition Effect Ring Oscillator (TERO) TRNG to active manipulation attacks. In particular we perform underpower and low temperature attacks on an implementation of the TERO running on a Xilinx Spartan 6 FPGA and experimentally evaluate the effectiveness of four online tests as countermeasure.