Gary McGraw wrote:
> Though I don't quite understand computer science theory in the same way that 
> Crispin does, I do think it is worth pointing out that there are two major 
> kinds of security defects in software: bugs at the implementation level, and 
> flaws at the design/spec level.  I think Crispin is driving at that point.
Kind of. I'm saying that "specification" and "implementation" are
relative to each other: at one level, a spec can say "put an iterative
loop here" and implementation of a bunch of x86 instructions. At another
level, specification says "initialize this array" and the implementation
says "for (i=0; i<ARRAY_SIZE;i++){...". At yet another level the
specification says "get a contractor to write an air traffic control
system" and the implementation is a contract :)

So when you advocate automating the implementation and focusing on
specification, you are just moving the game up. You *do* change
properties when you move the game up, some for the better, some for the
worse. Some examples:

    * If you move up to type safe languages, then the compiler can prove
      some nice safety properties about your program for you. It does
      not prove total correctness, does not prove halting, just some
      nice safety properties.
    * If you move further up to purely declarative languages (PROLOG,
      strict functional languages) you get a bunch more analyzability.
      But they are still Turing-complete (thanks to Church-Rosser) so
      you still can't have total correctness.
    * If you moved up to some specification form that was no longer
      Turing complete, e.g. something weaker like predicate logic, then
      you are asking the compiler to contrive algorithmic solutions to
      nominally NP-hard problems. Of course they mostly aren't NP-hard
      because humans can create algorithms to solve them, but now you
      want the computer to do it. Which begs the question of the
      correctness of a compiler so powerful it can solve general purpose

> If we assumed perfection at the implementation level (through better 
> languages, say), then we would end up solving roughly 50% of the software 
> security problem.
The 50% being rather squishy, but yes this is true. Its only vaguely
what I was talking about, really, but it is true.


Crispin Cowan, Ph.D.     
Director of Software Engineering
        AppArmor Chat:

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