The possibility for hardware trojans to be inserted into motherboards and processors – especially since many of these are manufactured abroad in countries such as China – has been a growing concern.
But, say the researchers in a paper titled 'Stealthy Dopant-Level Hardware Trojans' published last week, "since there have been no reported hardware trojans in practice yet, little is known about how such a trojan would look like, and how difficult it would be in practice to implement one." The purpose of their research, partly funded by the National Science Foundation, was to find out whether and how a hardware trojan could be implemented.
Their solution was not to 'add' a trojan to the hardware, but to create one "by changing the dopant polarity of existing transistors." Bruce Schneier explains: "Basically, you can tamper with a logic gate to be either stuck-on or stuck-off by changing the doping of one transistor." This can be done very late in the production cycle and makes no physical change to the circuit. The effect, say the researchers, is that "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.'"
The researchers demonstrate the effectiveness of their approach "by inserting Trojans into two designs," including the "cryptographically secure RNG design used in the [Intel] Ivy Bridge processors." This is the example that most intrigues Schneier. "Our Trojan is capable of reducing the security of the produced random number from 128 bits to n bits, where n can be chosen," say the researchers. "Despite these changes, the modified Trojan RNG passes not only the Built-In-Self-Test (BIST) but also generates random numbers that pass the NIST test suite for random numbers."
"This technique could," comments Schneier, "reduce the amount of entropy in Intel's hardware random number generator from 128 bits to 32 bits. This could be done without triggering any of the built-in self-tests, without disabling any of the built-in self-tests, and without failing any randomness tests." In other words, it substantially weakens any cryptography based on the Ivy Bridge RNG, without it ever being apparent.
"I have no idea if the NSA convinced Intel to do this with the hardware random number generator it embedded into its CPU chips, but I do know that it could... That's the worst thing about the NSA's actions. We have no idea whom we can trust."
Infosecurity asked Intel for a response to Schneier's comment, but was told that the relevant personnel "are currently at a conference."
One year ago, Intel commissioned Cryptography Research Inc to evaluate the Ivy Bridge Digital Random Number Generator. CRI found no evidence of a hardware trojan, and concluded, "the Ivy Bridge RNG is well designed, with a wide margin of safety, and the output is appropriate to use directly for cryptographic keys, secret nonces, and other sensitive values.”
But echoing Schneier's trust concern, there is always the hypothetical possibility that CRI tested one batch while Intel shipped another – this research demonstrates that both batches would appear to be identical.
Update
Intel has now contacted Infosecurity with the following statement:
"First, Intel does not participate in government efforts to decrease security in technology, and does not include backdoors for unauthorized access into its products.
"Second, the theory postulated in the paper is highly speculative and is based on a number of assumptions that are very challenging to overcome. Access to our manufacturing facilities, designs and other important data is strictly controlled."