Instead of adding additional circuitry to the target design, we insert our hardware Trojans by changing the dopant polarity of existing transistors. Since the modified circuit appears legitimate on all wiring layers (including all metal and polysilicon), our family of Trojans is resistant to most detection techniques, including fine-grain optical inspection and checking against "golden chips".
In a test of their technique against Intel's Ivy Bridge Random Number Generator (RNG) the researchers found that by setting selected flip-flop outputs to zero or one
Our Trojan is capable of reducing the security of the produced random number from 128 bits to n bits, where n can be chosen.
They conclude that
Since the Trojan RNG has an entropy of n bits and [the original circuitry] uses a very good digital post-processing, namely AES, the Trojan easily passes the NIST random number test suite if n is chosen sufficiently high by the attacker. We tested the Trojan for n = 32 with the NIST random number test suite and it passed for all tests. The higher the value n that the attacker chooses, the harder it will be for an evaluator to detect that the random numbers have been compromised.