  An Air Force evaluation of Multics, and Ken Thompson's 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 "trusting trust" attack goes undetected, even complete analysis of a
system's source code will not find the malicious code that is running.
Previously-known countermeasures have been grossly inadequate. If this attack
cannot be countered, attackers can quietly subvert entire classes of computer
systems, gaining complete control over financial, infrastructure, military,
and/or business systems worldwide. This dissertation's thesis is that the
trusting trust attack can be detected and effectively countered using the
"Diverse Double-Compiling" (DDC) technique, as demonstrated by (1) a formal
proof that DDC can determine if source code and generated executable code
correspond, (2) a demonstration of DDC with four compilers (a small C compiler,
a small Lisp compiler, a small maliciously corrupted Lisp compiler, and a large
industrial-strength C compiler, GCC), and (3) a description of approaches for
applying DDC in various real-world scenarios. In the DDC technique, source code
is compiled twice: the source code of the compiler's parent is compiled using a
trusted compiler, and then the putative compiler source code is compiled using
the result of the first compilation. If the DDC result is bit-for-bit identical
with the original compiler-under-test's executable, and certain other
assumptions hold, then the compiler-under-test's executable corresponds with
its putative source code.
