A Steganographic Covert Channel in the BB84 Classical Sifting Phase

This paper presents a steganographic covert channel embedded within the unencrypted, authenticated classical channel during the BB84 [1] sifting phase. Rather than altering the physical preparation of quantum states, risking the protocol’s Information-Theoretic Security (ITS), this channel utilizes “classical misreporting.” The sender maintains perfectly random quantum state preparation but intentionally falsifies specific basis announcements during sifting to transmit a covert payload. This method hides the covert data within the hardware’s expected Quantum Bit Error Rate (QBER) noise floor, controlled by a rolling trigger sequence, rendering it statistically indistinguishable from natural errors within the limited sample size of a standard session. The channel inflates the observable QBER by a factor that halves with each increment of the trigger length 𝑘, remaining well below the protocol’s abort threshold. The channel was empirically evaluated across trigger lengths 𝑘 = 6 through 𝑘 = 10 using Kolmogorov-Smirnov testing on both QBER distributions and inter-error distance distributions. At 𝑘 ≥ 10 the channel achieves statistical indistinguishability under both detection methods within 1,000 sessions in our experimental environment.