Most of the time we treat silicon chips—CPUs, for instance—as black boxes. The manufacturer publishes specifications for integrating it into a larger widget and making use of it, we code to those specifications, and the chip does its job. But inside the package is a fiendishly complicated machine, and there have been plenty of incidents where it didn’t work quite right, e.g. the infamous F00F and FDIV bugs in the Pentium. This is not to pick on Intel; every CPU manufacturer has had similar troubles, but only the x86 is sufficiently famous that its troubles make Wikipedia.

Anything that can happen by accident can happen on purpose. Most chip designers have to contract manufacture out to a fab plant run by a separate company, and the manufacturing process is opaque to them. It might occur to you to wonder whether the manufacturer can tamper with the design. It’s possible to disassemble a chip under an electron microscope and make sure all the wires are where they’re supposed to be, but this is expensive and destroys the chip, and it can only detect some of the possible changes to the design.

This paper presents a technique a fab plant could use to sabotage a chip design, that can’t be detected by any current technique for inspecting a chip after the fact. Basically, by changing the dopant mask, the fab can turn individual transistors into shorts to ground or V_cc. This is invisible to a microscope; all the wires are where they’re supposed to be, it’s just that a block of semiconductor has the wrong electronic properties. They demonstrate that this can be used to introduce subtle bugs that will not be caught by functional testing, such as making the output of a hardware RNG predictable, or introducing a power-consumption side channel into a cryptographic accelerator.

No solutions are presented; I’m not much of a hardware person and I have no idea what solutions might be possible. This is the hardware equivalent of a malicious compiler, and people are working on solving that problem—but they rely on the fact that you can inspect the output of a compiler in detail, because it’s just another string of bits. How do you detect that a block of silicon has been doped with boron instead of phosphorus? X-ray crystallography, maybe?