Post-Quantum Secure API Communication: Architectures and Migration Strategies for Quantum-Resilient Systems

This preprint presents a systems-security and architectural analysis of post-quantum cryptography deployment challenges in modern API ecosystems. The paper examines hybrid TLS 1.3 integration, cryptographic agility, migration strategies, API gateway architectures, certificate management, and operational considerations surrounding quantum-resilient distributed systems. The work focuses on applied cryptographic engineering and migration planning rather than proposing new cryptographic primitives. It synthesizes current standards, adversarial models, implementation constraints, and deployment considerations relevant to practical post-quantum migration for API infrastructure. Keywords: post-quantum cryptography, API security, TLS 1.3, ML-KEM, cryptographic agility, hybrid TLS.