  This paper presents a dual-factor authentication protocol and its low-power
implementation for security of implantable medical devices (IMDs). The protocol
incorporates traditional cryptographic first-factor authentication using
Datagram Transport Layer Security - Pre-Shared Key (DTLS-PSK) followed by the
user's touch-based voluntary second-factor authentication for enhanced
security. With a low-power compact always-on wake-up timer and touch-based
wake-up circuitry, our test chip consumes only 735 pW idle state power at 20.15
Hz and 2.5 V. The hardware accelerated dual-factor authentication unit consumes
8 $\mu$W at 660 kHz and 0.87 V. Our test chip was coupled with commercial
Bluetooth Low Energy (BLE) transceiver, DC-DC converter, touch sensor and coin
cell battery to demonstrate standalone implantable operation and also tested
using in-vitro measurement setup.
