Ken Thompson's famous "Reflections on Trusting Trust" (URL below) described a dangerous attack (often called the "trusting trust" attack) that subverts the system compiler. Up til now, it's been assumed that if an attacker succeeds, you are essentially at their mercy permanently there is no known detection technique, and countering methods have their own problems. David A. Wheeler has now written a paper on an approach to counter this attack.

The paper is called "Countering Trusting Trust through Diverse Double-Compiling" (URL below).

Here is an abstract:
An Air Force evaluation of Multics, and Ken Thompson's famous Turing award lecture "Reflections on Trusting Trust," showed that compilers can be subverted to insert malicious Trojan horses into critical software, including themselves. If this attack goes undetected, even complete analysis of a system's source code will not find the malicious code that is running, and methods for detecting this particular attack are not widely known. This paper describes a practical technique, termed diverse double-compiling (DDC), that detects this attack and some unintended compiler defects as well.

Simply recompile the purported source code twice: once with a second (trusted) compiler, and again using the result of the first compilation. If the result is bit-for-bit identical with the untrusted binary, then the source code accurately represents the binary.

This technique has been mentioned informally, but its issues and ramifications have not been identified or discussed in a peer-reviewed work, nor has a public demonstration been made. This paper describes the technique, justifies it, describes how to overcome practical challenges, and demonstrates it.

Related links: (Open in a new window.)
www.acm.org/classics/sep95/
www.acsa-admin.org/2005/abstracts/47.html
www.dwheeler.com/trusting-trust

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