Pure-DP Statistical Query Release at the Conjectured Square-Root Rate
Jack Fitzsimons
cs.DS
Jul 22, 2026 · v2
cs.CR
TL;DR
A companion Lean 4 development machine-checks the finite mechanism construction, its pure differential privacy after deterministic decoding, and the all-regimes error upper bound.
Abstract
Nikolov and Ullman asked whether k statistical queries on a universe of size T can be released under pure differential privacy with expected worst-coordinate error at the square-root rate suggested by known lower bounds. We prove their conjectured upper bound. For every database size n and privacy parameter $\varepsilon>0$, there is an $\varepsilon$-differentially private mechanism with expected error $O(\min\{1,\sqrt{\log(2T)\log(2k)/(\varepsilon n)}\})$. This matches the lower-bound dependence in the standard high-dimensional regimes where those bounds apply; the shifted logarithms and outer minimum make the upper bound valid without additional parameter assumptions. The construction starts from a selection-only private multiplicative weights transcript, then replaces its probability mass function by a distance-penalized likelihood envelope. To prove that the modification preserves accuracy, a likelihood-level Maurey argument upper-bounds each Hamming-ball maximum by a small family of auxiliary PMW laws. Renyi moment bounds control nearby balls, a direct mixture bound controls distant balls, and grouping radii at the privacy scale prevents an additional $1/\varepsilon$ factor in the error. The mechanism is information-theoretic. A companion Lean 4 development machine-checks the finite construction, pure privacy after deterministic decoding, and the displayed all-regimes upper bound.
Problem
Nikolov and Ullman asked whether k statistical queries on a universe of size T can be released under pure differential privacy with expected worst-coordinate error at the square-root rate sqrt(log T log k/(εn)). Classical pure-DP methods only achieve a cube-root rate.
Approach
The mechanism starts from a selection-only private multiplicative weights transcript that releases only signed query indices. It then replaces the transcript's probability mass function with a distance-penalized likelihood envelope, which is pure ε-DP by a triangle-inequality argument. Accuracy follows from a likelihood-level Maurey argument that dominates Hamming-ball maxima by small surrogate families. Rényi moment bounds handle nearby balls, mixture bounds handle distant balls, and radial blocking at width about 1/ε avoids an extra 1/ε factor. The finite construction, the privacy guarantee, and the final bound are machine-checked in Lean 4.
Results
There is an ε-DP mechanism with expected error O(min{1, sqrt(log(2T)log(2k)/(εn))}) for all parameters, with explicit constant C=2^20·e. This proves the conjectured upper bound, which matches known lower bounds in standard high-dimensional regimes.