constantTimeEquals function

bool constantTimeEquals(List<int> a, List<int> b)

Compares a and b for equality in constant time.

Returns true only if both have the same length and every element is equal. The comparison does not exit early on the first mismatching element, so its running time depends only on the length of the inputs and not on where they differ. This makes it safe for comparing secrets such as MACs and message digests, where an early return would leak how many leading bytes matched.

The length of the inputs is not treated as secret: a length mismatch returns false immediately. This is the only early return, and it branches only on the public lengths — never on the contents. Once the lengths match, every element is folded into an accumulator with no data-dependent branch, so the running time depends only on the length.

Parameters:

• a and b are the byte sequences to compare.

Implementation

bool constantTimeEquals(List<int> a, List<int> b) {
  int n = a.length;
  if (n != b.length) {
    return false;
  }
  int diff = 0;
  for (int i = 0; i < n; i++) {
    diff |= a[i] ^ b[i];
  }
  return diff == 0;
}