Hacker News Reader

Comment by nlewycky | Hacker News Reader

> I was bitten by one that injected timing attacks into certain integer operations by branching on the integer data in order to optimize 32-bit multiplications on 8-bit microcontrollers.

FWIW, I think this should be considered a language design problem rather than an optimizer design problem. Black box optimizer behaviour is good for enabling language designs that have little connection to hardware behaviour, and good for portability including to different extensions within an ISA.

C doesn't offer a way to express any timing guarantees. The compiler, OS, CPU designer, etc. can't even do the right thing if they wanted to because the necessary information isn't being received from the programmer.

View on HN

To a large degree, constant-time programming is hampered by the fact that even hardware is often unwilling to provide constant-time guarantees, let alone any guarantees that the compiler would care to preserve. (Although, to be honest, constant-time guarantees are the sort of things that most compiler writers prefer to explicitly not guarantee in any circumstances whatsoever).

If it were starting to show up in languages though, then that would make it more likely for hardware to start introducing functionality like this.

8 bit cpus offer constant time. 16 bit was starting to get into the issues where you cannot off it.

Your comment makes me wonder about the idea of building a superscalar, out of order, speculative implementation of the 6502 or 8080 instruction sets. Might make a good educational project.

Few languages provide such guarantees. But, there really was no way with this particular compiler to pass a hint to generate constant time code.

Black box designs work until the knob or dial you need to control it isn't there. I would have taken a pragma, a command-line option to the compiler, or even a language extension.

This is one example of many as to why I think that user-guided code generation should be an option of a modern tool suite. If I build formal specifications indicating the sort of behavior I expect, I should be able to link these specifications to the output. Ultimately, this will come down to engineering, and possibly, overriding or modifying the optimizer itself. An extensible design that makes it possible to do this would significantly improve my work. Barring that, I have to write assembler by hand to work around bad assumptions made by the optimizer.

Reply on news.ycombinator.com