I. Introduction
A physical unclonable function (PUF) can be embedded in a chip to take into account variations due to fabrication and thus produce a unique identification for the chip; so, it can be utilized to provide authentication in security applications. In this scheme, a set of die-unique responses are generated from a PUF circuit when a set of challenges are issued to that circuit; the relationship between the challenges and responses is determined by the variations. To date, quite a few PUF schemes and designs have been proposed, such as the arbiter PUF [1], the RO PUF [2], the SRAM PUF [3] and the butterfly PUF [4].