Function to count time with precision less than a millisecond - c++

Question

I have a function here that can make program count, wait etc with least count of 1 millisecond. But i was wondering if i can do same will lower precision. I have read other answers but they are mostly about changing to linux or sleep is guesstimate and whats more is those answers were around a decade old so maybe there might have come new function to do it.
Here's function-
void sleep(unsigned int mseconds)
{
clock_t goal = mseconds + clock();
while (goal > clock());
}
Actually, i was trying to make a function similar to secure_compare but i dont think it is wise idea to waste 1 millisecond(current least count) on just comparing two strings.
Here is function i made for the same -
bool secure_compare(string a,string b){
clock_t limit=wait + clock(); //limit of time program can take to compare
bool x = (a==b);
if(clock()>limit){ //if time taken to compare is more increase wait so it takes this new max time for other comparisons too
wait = clock()-limit;
cout<<"Error";
secure_compare(a,b);
}
while(clock()<limit); //finishing time left to make it constant time function
return x;
}

Answer 1

You're trying to make a comparison function time-independent. There are basically two ways to do this:
Measure the time taken for the call and sleep the appropriate amount
This might only swap out one side channel (timing) with another (power consumption, since sleeping and computation might have different power usage characteristics).
Make the control flow more data-independent:
Instead of using the normal string comparison, you could implement your own comparison that compares all characters and not just up until the first mismatch, like this:
bool match = true;
size_t min_length = min(a.size(), b.size());
for (size_t i = 0; i < min_length; ++i) {
match &= (a[i] == b[i]);
}
return match;
Here, no branching (conditional operations) takes place, so every call of this method with strings of the same length should take roughly the same time. So the only side-channel information you leak is the length of the strings you compare, but that would be difficult to hide anyways, if they are of arbitrary length.
EDIT: Incorporating Passer By's comment:
If we want to reduce the size leakage, we could try to round the size up and clamp the index values.
bool match = true;
size_t min_length = min(a.size(), b.size());
size_t rounded_length = (min_length + 1023) / 1024 * 1024;
for (size_t i = 0; i < rounded_length; ++i) {
size_t clamped_i = min(i, min_length - 1);
match &= (a[clamped_i] == b[clamped_i]);
}
return match;
There might be a tiny cache timing sidechannel present (because we don't get any more cache misses if i > clamped_i), but since a and b should be in the cache hierarchy anyways, I doubt the difference is usable in any way.