A Comparative Study of Classical and Post-Quantum Key Establishment: RSA vs. CRYSTALS–Kyber (ML-KEM)

The emergence of quantum computing poses an existential threat to classical cryptographic systems that form the backbone of contemporary digital security infrastructure. RSA, the most widely deployed public-key cryptosystem, relies on the computational hardness of integer factorization, a problem efficiently solvable by quantum computers using Shor's algorithm. This impending vulnerability has catalyzed the development of post-quantum cryptographic alternatives designed to withstand both classical and quantum attacks. CRYSTALS-Kyber, standardized by NIST as ML-KEM, represents the forefront of latticebased key encapsulation mechanisms, offering quantum resistance through the Module Learning With Errors (MLWE) problem. This paper presents a comprehensive comparative analysis of RSA and Kyber, examining their mathematical foundations, security architectures, operational characteristics, and performance metrics across diverse computational environments. The study investigates the fundamental trade-offs between legacy systems optimized for classical threat models and emerging quantum-resistant algorithms designed for future-proof security. Through detailed analysis of key generation, encryption/encapsulation, decryption/decapsulation operations, and security guarantees, this research establishes the technical rationale behind NIST's selection of Kyber as the primary post-quantum key establishment mechanism. The findings demonstrate that while RSA maintains advantages in key size efficiency, Kyber's superior performance profile, quantum resilience, and robust security foundation position it as the essential successor for securing digital communications in the quantum era. This comprehensive study provides critical insights for organizations transitioning from classical to post-quantum cryptographic infrastructure