Interface‐Level Transcript‐Assisted Leakage in Raw‐Feedback BB84‐State Quantum Identity Authentication

ABSTRACT

Quantum identity authentication (QIA) can provide quantum‐interface evidence for quantum secure communication, but its interaction with public transcript handling remains insufficiently characterized. This work analyzes a raw‐feedback QIA interface based on BB84 states and operating over an authenticated classical channel, where transcript integrity is protected but receiver feedback is public. First, we establish a no‐feedback benchmark: direct impersonation reduces to binary discrimination of value‐conditioned BB84 mixed states, for which unambiguous state discrimination is impossible and maximum‐confidence discrimination gives no forced‐response advantage over minimum‐error discrimination. Second, we identify a transcript‐conditioned leakage mechanism: when an adversary retains a quantum auxiliary system, public raw feedback can condition that system into a pure‐state discrimination instance for basis‐selection information. We quantify this leakage using guessing probability, conditional min‐entropy, accessible information, and an information‐disturbance tradeoff. Third, we separate leakage from exploitation: the leakage becomes an impersonation advantage only under implementation deviations such as key‐block reuse, weak session binding, abort‐handling failure, or overly permissive thresholds. These results yield conservative authentication‐length and threshold conditions for noisy and lossy optical implementations.