Quantum Hybrid PQC–QKD Security: Composable, Auditable Resilience for High-Value Transactions

Abstract High-value transactions—wholesale payments, securities settlement, energy trading, and sovereign digital services—depend on cryptographic assurances that must remain credible under discontinuous changes in adversary capability and under operational constraints that limit disruptive upgrades. This manuscript develops a composable, auditable security framework for hybrid deployment of post-quantum cryptography (PQC) and quantum key distribution (QKD). The central argument is that hybrid resilience is not achieved by deploying two technologies side-by-side; it is achieved by verified composition rules, migration algorithms that preserve business continuity, and governance practices that make assurance claims reviewable and contestable. The book integrates uncertainty-aware research design for long-horizon confidentiality and systemic dependencies, statistical modeling for explanation and causal inference on security and availability outcomes, and trustworthy machine learning for predictive assurance without undermining accountability. It further specifies scalable engineering controls for reproducible evidence, secure telemetry, and MLOps-like lifecycle management of cryptographic configurations. Research outputs are presented as reusable artifacts—composability claim templates, audit-ready evidence packets, hybrid key-lifecycle protocols, and decision tools for allocating upgrades and monitoring capacity—designed for application across South Asia, Europe, Africa, and the Americas without inventing datasets, institutions, or citations. Keywords post-quantum cryptography, quantum key distribution, hybrid security, composability, high-value transactions, key lifecycle, cryptographic agility, auditability, assurance case, uncertainty modeling, causal inference, trustworthy machine learning, robustness, secure telemetry, reproducibility, MLOps, governance artifacts, interoperability, migration protocol