Countering Trusting Trust through Diverse Double-Compiling

TL;DR

This paper introduces diverse double-compiling (DDC), a practical method to detect malicious compiler subversions like trusting trust, by recompiling source code twice with different compilers and comparing outputs.

Contribution

It formalizes and demonstrates the DDC technique, providing a peer-reviewed, practical solution for verifying compiler integrity and detecting malicious code insertions.

Findings

DDC can reliably detect trusting trust attacks.

The method is practical and can be implemented with existing tools.

Experimental results confirm the effectiveness of DDC.

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 compiler defects as well. Simply recompile the 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.