In the context of a system design, resource-constraints refer to severe restrictions on allowable resources, while resource-efficiency is the capability to achieve a desired performance and, at the same time, to reduce wasting resources. To design for low-cost platforms, these fundamental concepts are useful under different scenarios and they call for different approaches, yet they are often mixed. Resource-constrained systems require aggressive optimizations, even at the expense of performance, to meet the stringent resource limitations. On the other hand, resource-efficient systems need a careful trade-off between resources and performance, to achieve the best possible combination. Designing systems for resource-constraints with the optimizations for resource-efficiency, or vice versa, can result in a suboptimal solution. Using modern cryptographic applications as the driving domain, I first distinguish resource-constraints from resource-efficiency. Then, I introduce the recurring strategies to handle these cases and apply them on modern cryptosystem designs. I illustrate that by clarifying the application context, and then by using appropriate strategies, it is possible to push the envelope on what is perceived as achievable, by up to two orders-of-magnitude. In the first part of this dissertation, I focus on resource-constrained modern cryptosystems. The driving application is Physical Unclonable Function (PUF) based symmetric-key authentication. I first propose the smallest block cipher in 128-bit security level. Then, I show how to systematically extend this design into the smallest application-specific instruction set processor for PUF-based authentication protocols. I conclude this part by proposing a compact method to combine multiple PUF components within a system into a single device identifier. In the second part of this dissertation, I focus on resource-efficient modern cryptosystems. The driving application is post-quantum public-key schemes. I first demonstrate energy-efficient computing techniques for post-quantum digital signatures. Then, I propose an area-efficient partitioning and a Hardware/Software codesign for its implementation. The results of these implemented modern cryptosystems validate the advantage of my approach by quantifying the drastic improvements over the previous best.

Ph. D.