Mixed-Signal Physically Unclonable Function With CMOS Capacitive Cells

An electronic physically unclonable function usually includes an on-chip error-correcting code unit, which is vulnerable to security attacks and adds area, power, and data processing time overheads. This paper proposes a mixed-signal physically unclonable function circuit for authentication purposes, which we call the enhanced capacitive physically unclonable function. It divides the input challenge word over multiple computational groups to decrease processing time, increase security, and eliminate the need for error-correcting code units. Most of the challenge bits control capacitive networks grouped into several capacitive cells, while some are analogized through two digital-to-analog converters. One digital-to-analog converter controls the discharge loads of the capacitive cells; the other controls the reference voltage of comparator units. Each comparator controls a counter that digitizes the discharge time into a response chunk. Most of these counters operate at high frequencies for more precise time-to-digital conversion and are overflown to act as roulettes to promote unpredictability. One counter is not overflown to generate a reference response chunk to support error handling. The design allows for more intrinsic variations throughout the fabrication process, leading to unique response chunks. It applies an expanding challenge-response pair approach, generating a 128-bit response word for a 64-bit challenge word. The capacitive nature of the design supports various security features. Simulating the circuit using 45 nm complementary metal-oxide semiconductor technology resulted in an average power of 921.67 μW, a layout area of 22,470 μm
²
, and an average data processing time of 118 μs.