Lightweight encryption circuits are crucial to ensure adequate information security in emerging millimeter-scale platforms for the Internet of Things, which are required to deliver moderately high throughput under stringent area and energy budgets. This requires the adoption of specialized AES accelerators, as they offer orders of magnitude energy improvements over microcontroller-based implementations. In this paper, we present the architectural exploration of lightweight AES accelerators with the goal of minimizing the energy consumption. Also, the lower bound of the number of cycles per encryption in lightweight AES designs is estimated as a function of the number of available S-boxes. Combined with sub-/near-threshold circuit techniques, we present a low-cost ultra energy-efficient AES encryption core for cubic-millimeter platforms. Our test chip achieves high energy efficiency of 0.83 pJ/bit at 0.32V, which outperforms the state-of-the-art low-cost AES designs by 7×.