Wireless implantable medical devices are expected to perform cryptographic processing at an absolutely low level of power consumption. This paper presents the design of an ultra-low power ASIC core implementing the PRESENT encryption algorithm. To minimize power consumption, subthreshold CMOS logic is adopted. To implement robust combinational logic (S-Boxes) in PRESENT at subthreshold, a multiplexor-tree architecture based on CMOS transmission gates is proposed. Our post-layout simulations show that our PRESENT core consumes around 50 nW at 0.35V supply voltage at 25 kHz clock frequency, proving the feasibility of ultra-low power encryption.