Quantum-Resistant Secure Communication System

Abstract - Most secure messaging today relies on RSA or elliptic-curve Diffie–Hellman, which a sufficiently large quantum computer could break, and traffic recorded today can be stored and decrypted later. This paper describes a multi-client secure communication system that uses post-quantum cryptography for session-key establishment. Key encapsulation uses ML-KEM-768 (FIPS 203) by default, identities and key-distribution packets are signed with ML-DSA-65 (FIPS 204), and message and file contents are protected with AES-256-GCM. The relay server routes traffic and metadata but is not a cryptographic authority: a receiver verifies a packet's signature against its own locally held key before decapsulating or installing any key, and a peer is trusted only after explicit out-of-band confirmation. We evaluated the system with 200-iteration micro-benchmarks, 2,188 automated tests in 15 batches, 33 UI-driven acceptance tests and six attack and fault scenarios. In this implementation, a full MLKEM-768 session-key round trip takes 8.16ms against 0.51ms for RSA-2048, a gap that mainly reflects a pure-Python MLKEM library compared with native RSA. ML-KEM-768 public keys and ciphertexts are 4.6 and 4.3 times larger. ML-DSA65 signing averages 0.62ms. Every forged, stripped, malformed or tampered input was rejected before any key was installed. Manual acceptance testing and real-hardware audio checks remain open.