e-NFC: A Hardware-Enforced Mutual Authentication Architecture for ULP Wearables Using Capacitive EQS-HBC

Wearable authentication for μW-scale, charging-free body nodes faces fundamental security challenges because conventional wireless links are radiative and inherently exposed, while demonstrated cryptographic solutions often exceed the available power budget. As a result, many ultra-low-power wearables remain vulnerable to passive observation, replay, and unauthorized activation. Emerging wearable applications increasingly mediate sensitive functions such as access control, identity verification, and medical data exchange, necessitating stronger security guarantees under severe energy constraints. This work presents an energy-efficient authentication architecture that addresses this gap by combining a spatially confined capacitive electro-quasistatic human body communication (EQS-HBC) physical layer with a lightweight protocol controller and a fully integrated AES-256 cryptographic core. This work provides a silicon-demonstrated, μW-class secure authentication tag that completes an end-to-end NFC-style mutual authentication over the EQS-HBC channel. The proposed e-NFC system enables NFC-style mutual authentication under a sub-10 μW power envelope without relying on radiative RF signaling. Implemented in 65 nm CMOS, the prototype achieves nanowatt-scale idle power through aggressive power gating and completes mutual authentication within 80 ms. Measurement results demonstrate robust resistance to passive and active wireless threats while remaining compatible with long-term, charging-free wearable operation.