Physical unclonable function (PUF), sometimes also called physically unclonable function, is a physical entity that is embodied in a physical structure and is easy to evaluate but hard to predict. All PUFs are subject to environmental variations such as temperature, supply voltage and electromagnetic interference, which can affect their performance. Therefore, rather than just being random, the real power of a PUF is its ability to be different between devices, but simultaneously to be the same under different environmental conditions.