This paper presents a lightweight cryptographic system integrated onto a multi-function implantable biosensor prototype. The resulting heterogeneous system provides a unique and fundamental capability by immediately encrypting and signing the sensor data upon its creation within the body. By providing these security services directly on the implantable sensor, a number of low-level attacks can be prevented. This design uses the recently standardized SHA-3 Keccak secure hash function implemented in an authenticated encryption mode. The security module consists of the DuplexSponge security core and the interface wrapper. The security core occupies only 1550 gate- equivalents, which is the smallest authenticated encryption core reported to date. The circuit is fabricated using 0.18 μm CMOS technology and uses a supply voltage of 1.8 V. The simulated power consumption of the complete cryptosystem with a 500 KHz clock is below 7 μW.