To protect large-scale power-grid critical infrastructures, efficient security and intrusion prevention techniques are required to prevent remote adversaries from attacking the computational and data resources. Traditionally, trusted computing base solutions, i.e., a small easy-to-verify software and/or hardware component, act as the trust root for the overall execution. In this paper, we present Trusted Sensing Base (TSB), a new data trust root solution for power grid infrastructures. In particular, TSB is attached to exact data acquisition points, where sensors obtain power measurements. TSB's main responsibility is to encrypt analog power current and voltage signals, and feed the sensors, such as phasor measurement units (PMUs), with encrypted signals. Therefore, the proposed TSB solution prevents remote adversarial parties from accessing or modifying the acquired data on a sensor without being detected even if they succeed in full system penetration and completely compromise of the sensor. We have implemented a complete hardware working prototype of TSB, and evaluated it on a real-world power grid testbed infrastructure. Our experimental results show that TSB significantly improves the power grid security with minimal deployment and performance overhead on the power grid operations.