Body area networks (BANs) require lightweight session key establishment, yet public key exchange imposes computation and energy costs that exceed the budgets of deeply constrained wearable nodes. This brief presents HHK, a hardware-oriented cross-location photoplethysmography (PPG) key generation architecture for BANs. The proposed datapath extracts inter-beat intervals (IBIs) from green-light PPG at multiple body sites, aligns beat timestamps across locations, applies Gray-coded equal-frequency quantization, and employs a rate-1/3 polar code fuzzy commitment (N=128, K=42) to reconcile residual timing mismatches. Post-implementation synthesis on a Xilinx XC7Z020 maps the complete datapath to 18,760 lookup tables and 20,971 flip-flops with no multipliers or embedded memories, giving 48 uJ per key generation event and 0.4 uW average power over a 120-second acquisition window. Validation across 16 participants from a real ambulatory dataset (216 hours; head, wrist, and ankle) yields 86.0-90.1% raw IBI bit agreement and 51.3-69.8% key agreement. To the best of our knowledge, HHK is the first synthesizable register-transfer level (RTL) key generation architecture for BANs validated across multiple body locations on ambulatory data.