  Privacy and energy are primary concerns for sensor devices that offload
compute to a potentially untrusted edge server or cloud. Homomorphic Encryption
(HE) enables offload processing of encrypted data. HE offload processing
retains data privacy, but is limited by the need for frequent communication
between the client device and the offload server. Existing client-aided
encrypted computing systems are optimized for performance on the offload
server, failing to sufficiently address client costs, and precluding HE offload
for low-resource (e.g., IoT) devices. We introduce Client-aided HE for Opaque
Compute Offloading (CHOCO), a client-optimized system for encrypted offload
processing. CHOCO introduces rotational redundancy, an algorithmic optimization
to minimize computing and communication costs. We design Client-Aided HE for
Opaque Compute Offloading Through Accelerated Cryptographic Operations
(CHOCO-TACO), a comprehensive architectural accelerator for client-side
cryptographic operations that eliminates most of their time and energy costs.
Our evaluation shows that CHOCO makes client-aided HE offloading feasible for
resource-constrained clients. Compared to existing encrypted computing
solutions, CHOCO reduces communication cost by up to 2948x. With hardware
support, client-side encryption/decryption is faster by 1094x and uses 648x
less energy. In our end-to-end implementation of a large-scale DNN (VGG16),
CHOCO uses 37% less energy than local (unencrypted) computation.
