为什么我们需要一个恒定的时间*单字节*比较功能？

Question

Looking at Go standard library, there's a ConstantTimeByteEq function that looks like this:

func ConstantTimeByteEq(x, y uint8) int {
    z := ^(x ^ y)
    z &= z >> 4
    z &= z >> 2
    z &= z >> 1

    return int(z)
}

Now, I understand the need for constant time string (array, etc.) comparison, as a regular algorithm could short-circuit after the first unequal element. But in this case, isn't a regular comparison of two fixed-sized integers a constant time operation at the CPU level already?

Answer 1

Not necessarily. And it is hard to tell what the compiler will emit after doing its optimizations. You could end up with different machine code for the highlevel "compare one byte". Leaking just a tiny bit in a side channel might change your encryption from "basically unbreakable" to "hopefully not worth the money needed for a crack".