Cross-layer downgrade-resistant security negotiation for the quantum Internet protocol stack

The quantum internet is evolving from isolated point-to-point quantum key distribution links toward a globally interconnected, multi-layer protocol stack supporting entanglement distribution, quantum teleportation, and distributed quantum computing. As this architecture matures, each protocol layer-physical, link, network, transport, and application exposes negotiable security parameters: QKD protocol type, entanglement fidelity thresholds, purification rounds, error-correction schemes, and authentication methods. Yet no standardized mechanism exists to negotiate these parameters securely across layers or to prevent adversarial downgrade. The classical internet spent two decades rediscovering this lesson through attacks such as POODLE, FREAK, Logjam, KNOB, and Dragonblood; the quantum internet is now repeating the same architectural oversight. This study introduces QShield , a cross-layer security negotiation framework that binds every layer’s capability commitment into a unified transcript hash, ensuring that any single-layer downgrade invalidates the session-wide security context. We formalize a quantum security downgrade hierarchy spanning six levels from device-independent QKD to classical cryptography and identify twelve concrete downgrade attack vectors mapped from classical analogs. Evaluation across all twelve attack types shows that QShield with cross-layer transcript binding achieves 100% deterministic detection across the twelve modeled downgrade vectors evaluated under emulation, whereas independent per-layer checks detect only 40–60% of coordinated multi-layer attacks. Hybrid mode preserves near-classical session bootstrap latency (48-55 ms versus 25-30 ms baseline), while the signaling overhead remains within practical bounds (3.5-5 KB versus 0.8-1.2 KB for classical).