Since at least the ’70s, the time of Multics (see eg. this old document on the vulnerability analysis of Multics security), the Orange Books, Military IT security etc., the role of hardware in IT security has been discussed, evaluated and implemented.
In the last years the discussion has risen again in particular about the possibility of hardware backdoors and malicious hardware. For example, since the publication of the Snowden documents there have been rumors about possible hardware backdoors in Intel, AMD and Cisco products.
A few days ago at the 2016 IEEE Symposium on Security and Privacy has been presented this paper (see eg. also here for a summary) describing how to implement a Hardware Backdoor called Analog Malicious Hardware which, as of today, seems practically impossible to detect. The researchers were able to add a tiny circuit composed by a capacitor and a few transistors wrapped up in a single gate, out of the millions or billions in a modern chip, which acts as the hardware Trojan horse.
How difficult could it be to add a single, almost undetectable gate to the blue prints of a chip at the chip factory? How can be verified that similar gates are not present on a chip?
PS. 10 years ago I gave a couple of seminars in Italian about some aspects of history of IT security and I looked into some issues of how hardware must support the security features of Operating Systems; if interested some slides and a paper (in Italian) can be found here and here.