Secure Resource Allocation Against Honest-but-Curious Relays in Underlay CR-NOMA Networks: A Constraint-Aware Hybrid D3QN–TD3 Approach

Underlay cognitive radio non-orthogonal multiple access (CR-NOMA) enables secondary spectrum reuse under primary-user quality-of-service (QoS) protection, and cooperative amplify-and-forward (AF) relaying extends coverage for edge secondary users. However, third-party relay-capable user equipment may behave as honest-but-curious AF relays that follow the prescribed forwarding protocol while attempting to infer confidential secondary messages. To suppress relay-side eavesdropping, this paper proposes a destination-assisted jamming-based secure resource allocation framework in which the far secondary user transmits an artificial-noise sequence unavailable to the selected relay. A constraint-aware hybrid algorithm combining a dueling double deep Q-network (D3QN) with twin delayed deep deterministic policy gradient (TD3) is developed to jointly optimize relay selection and continuous power allocation under primary-user QoS, secondary-user QoS, successive interference cancellation (SIC) feasibility, and transmit-power constraints, where an adaptive decayed constraint-penalty mechanism balances secrecy enhancement and constraint satisfaction. On held-out test traces, the proposed controller achieves an effective secrecy sum rate of 0.8099±0.0219 bps/Hz with 90.68% mean full-constraint feasibility over five training seeds. This rate is 96.6% and 48.8% higher than those of standalone TD3 and deep deterministic policy gradient (DDPG), respectively, while the projected-dual and adapted D3QN–DDPG baselines attain comparable sample means. These results demonstrate the effectiveness of the proposed framework for constraint-aware secure resource allocation in dynamic relay-assisted underlay CR-NOMA networks.