Metastability occurrence based physical unclonable functions for FPGAs

A novel concept of a physical obfuscation of cryptographic keys is introduced which basically utilises identification of the metastable behaviour of field programmable gate array (FPGA) flip-flops, i.e. the time variability of the metastability´s appearance. Clock and data intervals leading to metastability (δ) are measured with the use of programmable delay lines available in digital clock managers (DCMs). The DCM acts as a variable clock phase regulator, whereas another circuit detects metastable events. The parts of binary words representing δ become segments of a physical unclonable function key. Preliminary results show significant differences in interval δ values when a slight change in circuit implementation occurs (in placement, node connections, surrounding sub-circuits etc.). Similar FPGA devices manufactured in the same process having an identical structure also behave significantly differently due to the intrinsic inter-class randomness.