I’m writing up a short summary for each classic cybersecurity paper that I have to know for my qualification exam. This week, let’s chat about “Reflections on Trusting Trust” by Ken Thompson (1984).

The three page paper comes from Ken Thompson’s Turing Award lecture in 1984. In it, he details a very elegant attack by which a backdoor can be injected into a program through a malicious compiler, leaving no evidence in the actual source code of the compiler or the program.

Moral

The moral is obvious. You can’t trust code that you did not totally create yourself.

Thompson argues that a skillful attacker can install a bug that will be almost impossible to detect. He spends the rest of the speech to say that most unauthorized access to computer systems is vandalism and should be treated as such by the media, the law, and society at large. This drawn out emphasis on the treatment of vandals undercuts the seriousness of the previous attack.

The moral is not that we cannot trust code we did not write ourselves.  The moral is that even the code we write could be corrupted. The keyword in Thompson’s paper is “totally.” This turtles-all-the-way-down attack stops only when you use your own vacuums.

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