Structural Vulnerability in Y00 Protocols

This paper critically analyzes the Y00 protocol, a quantum noise-based stream cipher proposed to enhance classical cryptographic methods through quantum mechanical properties. Despite its promise, we reveal a structural vulnerability that enables the leakage of secret information from measurement outcomes. To systematically evaluate its security, we first formalize the claims of previously proposed Y00 protocols, clarifying their achievements and limitations. We then identify the structural vulnerability through an intuitive explanation and rigorous formulation using maximum likelihood estimation. Our findings demonstrate that Y00's structural weaknesses allow for the unique determination of the shared secret, leading to significant information leakage. Using the "Toy protocol" as a reference model, we contextualize these results within the broader field of security technology. Furthermore, we generalize our findings to a wider class of quantum-based stream cipher protocols, identifying a fundamental security condition that Y00 fails to satisfy. This condition serves as a critical benchmark for ensuring the security of any stream cipher protocol relying on physical states, whether quantum or classical. These findings underscore the importance of rigorous security evaluations, particularly in systems intended for practical applications. Unexamined vulnerabilities not only undermine trust but also expose systems to avoidable risks, making rigorous analysis indispensable for ensuring resilience and security.