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.