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RQ2-P3 mechanism study (NEW prereg, approved-path/params-locked, not yet frozen):
pool-based willing-bridge instrument, concentration swept as a manipulated IV
(B∈{2,4,8}×α∈{0,1,2}), two-sided/direction-agnostic, run-level cluster bootstrap.
RQ3 companion run-brief (execution-freeze for the already-frozen RQ3): agent=qwen2.5:3b
local Ollama, churn kill_prob=30/steps=20, baselines + Holm-7 completion pinned.
note-unique-bridge-artifact.md: on-record analysis that the lead RQ2-P1 shrink is plausibly
a fresh-bridge-per-circuit artifact (unique exit-signature -> singleton set -> H~0), and that
realistic bridge reuse (a mix) may raise anonymity -> the mechanism study may qualify the lead
RQ2 headline. No edit to the frozen prereg or committed lead paper.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
3.3 KiB
Note — the RQ2-P1 "shrink" is plausibly a unique-bridge instrument artifact
Status: on-record analysis note, 2026-07-21. Does not edit the frozen prereg or the
committed lead paper; it documents a mechanism concern for the record and motivates the
rq2p3-mechanism-prereg.md follow-up. Author: overseer, post-lead-paper review.
The finding
The lead paper (RQ2-P1) reports federation shrinks the per-circuit anonymity set (ΔH = −0.96 bits pooled; −3.63 bits bridge-federated-only, exploratory). Tracing the ratified posterior mechanically shows this shrink is largely forced by the instrument, not by a substantive funnelling phenomenon:
- The adversary's observation is
exit_signature = (exit_house, bridge_label)(confirm_load_rq2.py:exit_signature). - The anonymity set for circuit i is the distinct entry nodes among circuits sharing
i's signature (
observation_consistent_sizes):m_i = |{entry(j) : sig(j) == sig(i)}|. - The bridge-federated instrument assigns a fresh bridge per circuit seed
(
assembler.py:_bridge_label(seed)), so every circuit'sbridge_labelis unique ⇒ everyexit_signatureis unique ⇒ each group has size 1 ⇒m_i = 1⇒H_i ≈ 0.
So bridge-federated collapses to near-zero entropy because each circuit was handed its own bridge, making its signature a unique identifier. That is the same degeneracy that made RQ2-P3 untestable (zero-variance concentration) — here it manifests as an artificially low H that drives ΔH negative.
Why this likely inverts under realistic bridge reuse
Introduce a finite shared bridge pool: many circuits share a bridge ⇒ shared signature ⇒
m_i = distinct entries across all of them ⇒ larger set ⇒ higher H. Under this posterior
a shared bridge behaves as a mix: more concentration plausibly raises anonymity — the
opposite of the naive "funnelling shrinks the set" story. The mechanism study
(rq2p3-mechanism-prereg.md) tests this two-sided; the mechanical prediction is mix
(ρ > 0), the naive prediction is funnel (ρ < 0).
Consistency check against observed numbers
- Directory-federated cell (no bridge hop →
bridge_label = None, shared signature): observedrq2_anonymity_entropy_bits = 5.6439 = log₂(50)— maximal entropy, all 50 senders in the set. Confirms shared signatures give large sets. - Bridge-federated (unique bridge per circuit): H ≈ 0, consistent with the exploratory ΔH = −3.63. The pooled −0.96 is the average of a high-H directory arm and a ≈0-H bridge arm.
Integrity status
- The lead paper is not wrong or dishonest: §7 already flags RQ2 as conditional on the posterior construction and RQ2-P3 as an unresolved mechanism. This note sharpens that caveat from "conditional" to "the specific shrink magnitude is mechanically forced by the fresh-bridge instrument."
- No silent edit to the committed paper or frozen prereg. If the mechanism study confirms mix (ρ > 0), the honest outcome is a companion result that qualifies/corrects the lead RQ2-P1 headline — reported openly, per the null-results-are-results rail.
- This strengthens the body of work: it is exactly the kind of artifact a pre-registered, frozen-detector method is supposed to expose rather than bury.