[TUHS] Thompson trojan put into practice

[Douglas McIlroy]

> It's part of my academic project to work on provable compiler security.
> I tried to do it according to the "Reflections on Trusting Trust" by Ken
> Thompson, not only to show a compiler Trojan horse but also to prove that
> we can discover it.

Of course it can be discovered if you look for it. What was impressive about
the folks who got Thompson's compiler at PWB is that they found the horse
even though they weren't looking for it.

Then there was the first time Jim Reeds and I turned on integrity control in
IX, our multilevel-security version of Research Unix. When it reported
a security
violation during startup we were sure it was a bug. But no, it had snagged Tom
Duff's virus in the act of replication. It surprised Tom as much as it did us,
because he thought he'd eradicated it.

Doug