You assume that boolean operations are constant time, and whether that holds depends on the uarchitecture and how sophisticated the optimization layers are (e.g. nothing prevents the compiler or even the CPU from short-circuiting the OR as soon as the first XOR is non-zero).
Computing a MAC of the two input values under a once-off random key is actually much stronger.
As a matter of that, this highlights that the goal is to lower the SNR for the attacker, and constant time computation is only one of the two non-exclusive ways to achieve that, the other being adding sufficient noise to their measurements.
Computing a MAC of the two input values under a once-off random key is actually much stronger.
As a matter of that, this highlights that the goal is to lower the SNR for the attacker, and constant time computation is only one of the two non-exclusive ways to achieve that, the other being adding sufficient noise to their measurements.