  During the last years, Physically Unclonable Functions (PUFs) have become a
very important research area in the field of hardware security due to their
capability of generating volatile secret keys as well as providing a low-cost
authentication. In this paper, an introduction to Physically Unclonable
Functions is given, including their definition, properties and applications.
Finally, as an example of how to design a PUF, the general structure of a ring
oscillator PUF is presented.
