Auditing Combinatorial Randomness from Finite Transcripts

Abstract

Public randomness is a security primitive whose deployed behavior is often observable only through a finite transcript. We study black-box auditing of $k$-subset draws from $m$ labels under the exact uniform-without-replacement null. The outcome space has size $\binom{m}{k}$, and unrestricted uniformity testing therefore requires $\Theta(\sqrt{\binom{m}{k}}/\varepsilon^2)$ samples, establishing an information-theoretic limit on transcript-only certification. For structured faults, we construct generator-agnostic audits on the hypersimplex using marginal chi-square, pair maxima, serial overlap, anchored-box discrepancy, and low-dimensional $H_0$/MST geometry, all calibrated under the exact combinatorial null. We also prove a finite-witness guarantee whose sample complexity depends logarithmically on the number of audited witnesses rather than on the full support size. Across observed and reference-source audits, no statistic remains significant after false-discovery correction (minimum BH $q=0.279$). GPU Monte Carlo experiments, using up to 300,000 null and 60,000 alternative replications per condition, show that marginal-preserving deviations can evade one-dimensional tests while remaining detectable through joint geometry. At $n=1{,}956$, a block-cluster alternative of strength 0.04 yields power 0.638 for pair maxima versus 0.051 for marginal chi-square; a band-repulsion alternative of strength 0.08 yields power 0.741 for anchored boxes versus 0.051. These results characterize which structured deviations finite public transcripts can detect and the sample sizes required for doing so.