oak_techniques: [OAK-T14.005]
spec_id: oak-detection-T14.005
version: 0.1.0
maturity: emerging
maintainer: "@iZonex"
license: Apache-2.0

scope: |
  Detect builder-level transaction censorship in proposer-builder separation
  (PBS) architectures where block builders deliberately exclude specific
  transactions from their block proposals to benefit their own or allied
  searchers' transaction flows. Covers both profit-driven censorship
  (excluding rival MEV bundles) and compliance-driven censorship (excluding
  OFAC-sanctioned interactions). Detection operates at the per-builder
  transaction-inclusion audit and builder-searcher correlation layers.
  Excludes: T14.002 (MEV-Boost relay attack — relay-layer manipulation,
  not builder-layer); T5.004 (sandwich MEV — per-transaction pattern,
  not builder-censorship infrastructure).

data_sources: [beacon_proposer_assignments, mempool_trace,
               tx_call_trace, dex_trades]

detection_logic:
  description: |
    Four detection paths. PATH A (per-builder transaction-inclusion audit):
    for each builder, compute the set of searcher addresses whose transactions
    appear in the builder's blocks; compare across builders; flag builders
    whose inclusion set materially deviates from the global top-N-searchers
    set. PATH B (per-transaction inclusion-delay monitoring): track
    transactions observed in the public mempool; alert on transactions that
    remain un-included beyond a configurable slot-count threshold while
    paying competitive priority fees. PATH C (builder-allied-searcher
    correlation): identify searcher addresses appearing exclusively in a
    specific builder's blocks. PATH D (OFAC-compliance censorship): track
    inclusion rates of transactions interacting with OFAC-sanctioned
    addresses per builder.
  pseudocode: |
    # PATH A — per-builder transaction-inclusion audit
    for each builder B:
      B_searchers ← {addr for tx in B.blocks if tx.from ∈ known_searchers}
      global_searchers ← {addr for tx in all_blocks if tx.from ∈ known_searchers}
      missing ← global_searchers − B_searchers
      if len(missing) / len(global_searchers) > builder_exclusion_threshold:
        emit(PATH_A, builder=B, excluded_searchers=missing,
             exclusion_ratio=len(missing)/len(global_searchers), severity=high)

    # PATH B — per-transaction inclusion-delay monitoring
    for each tx T in public_mempool:
      slots_pending ← current_slot − T.first_seen_slot
      if slots_pending > inclusion_delay_threshold
         and T.priority_fee >= competitive_fee_percentile:
        included_by ← [B for B in builders if T.hash ∈ B.blocks]
        if included_by == ∅:
          emit(PATH_B, tx=T.hash, from=T.from, to=T.to,
               slots_pending, priority_fee=T.priority_fee,
               severity=high)

    # PATH C — builder-allied-searcher correlation
    for each builder B:
      B_only ← [S for S in B_searchers
                 if S.appears_in(B.blocks) / S.total_appearances > exclusivity_ratio]
      for each searcher S in B_only:
        mev_share ← S.mev_extracted_in(B.blocks) / B.total_mev
        if mev_share > builder_searcher_mev_threshold:
          emit(PATH_C, builder=B, searcher=S, exclusivity=S.appears_in(B.blocks)/S.total_appearances,
               mev_share, severity=medium)

    # PATH D — OFAC-compliance censorship
    for each builder B:
      sanctioned_txs ← [T for T in mempool where T.interacts_with(ofac_sdn_addresses)]
      B_included ← [T for T in sanctioned_txs if T.hash ∈ B.blocks]
      inclusion_rate ← len(B_included) / len(sanctioned_txs)
      if inclusion_rate == 0 and len(sanctioned_txs) > min_sanctioned_sample:
        emit(PATH_D, builder=B, inclusion_rate=0,
             sanctioned_tx_count=len(sanctioned_txs), severity=medium)

parameters:
  builder_exclusion_threshold:       { type: number,   default: 0.2 }
  inclusion_delay_threshold:         { type: integer,  default: 6 }        # slots
  competitive_fee_percentile:        { type: number,   default: 50 }
  exclusivity_ratio:                 { type: number,   default: 0.8 }
  builder_searcher_mev_threshold:    { type: number,   default: 0.1 }
  min_sanctioned_sample:             { type: integer,  default: 10 }
  ofac_sdn_addresses:                { type: list,     default: [] }

output_alert: [oak_technique, detection_path, severity, chain,
               builder, searcher, excluded_transactions, inclusion_delay,
               censorship_type, evidence]

test_fixtures:
  positive:
    - 2022-2025-tornado-cash-ofac-builder-censorship                    # OFAC-compliance censorship, extensively measured
    - 2022-2025-ethereum-block-builder-eof-centralization               # Builder-allied searcher exclusivity
  negative:
    - "Builder with below-average inclusion set matching its market share — no statistically significant exclusion pattern"
    - "Transaction delayed by network congestion rather than builder censorship — inclusion delay without builder-specific exclusion pattern"

false_positive_modes:
  - Builder concentration is a structural feature of PBS — per-builder inclusion-set variance from a small builder is noise, not censorship; calibrate PATH A thresholds per builder market share
  - Transaction inclusion delay from network congestion or low priority fee — PATH B must verify competitive fee percentile before flagging
  - Builder-allied searcher exclusivity from private order flow (searcher submits bundles exclusively to one builder) — this is structural PBS behaviour, not necessarily malicious; PATH C is a transparency metric, not a detection alert
  - OFAC-compliance censorship is a regulatory compliance decision, not an "attack" — PATH D is an observability metric; treat as a transparency signal, not a detection incident

mitigations: [OAK-M09]

reference_implementations:
  - { target: eigenphi,               chain: evm,    url: "" }
  - { target: censorship-dot-pics,     chain: evm,    url: "" }
  - { target: mevwatch-dot-info,       chain: evm,    url: "" }
  - { target: relayscan,              chain: evm,    url: "" }