Comment by deadfa11

Basically, measure the time difference in the memory comparison. Typically, comparisons will short-circuit. That is, stop as soon as a difference is detected. But this is a problem for security situations like key comparisons. If 10 out of 20 bytes match, it will take a little bit longer to compare then if 5 out of 20 bytes match. With enough iterations, the key can be recovered due to these differences in time. The correct solution is to use a constant time comparison that always runs through the entire sequence of bytes.