Skip to content

Bug: DKG permanent failure — dealer consensus uses unweighted participant votes but quorum uses slot weights #849

Closed @Mayveskii opened 2026-03-03 12:04 UTC 2 comments Updated 2026-03-12 22:56 UTC

Location

inference-chain/x/bls/keeper/phase_transitions.go — lines 74 and 295–318

Description

The DKG pipeline uses two independent threshold checks with different weighting schemes that are fundamentally inconsistent:

// 1. Transition to VERIFYING — slot-weighted quorum (correct)
if slotsWithDealerParts > epochBLSData.ITotalSlots/2 { ... }

// 2. Dealer consensus — unweighted (count of participants, NOT slots)
dealerIsValid := totalVotes > 0 && validVotes > totalVotes/2

Concrete failure scenario

Suppose: - Participant A holds 60% of total slots - Participants B, C, D each hold ~13% of slots

  1. A submits dealer parts → slotsWithDealerParts > ITotalSlots/2 → DKG transitions to VERIFYING ✓
  2. B, C, D vote A's dealer invalid (3 vs 1 participant votes → majority by count)
  3. A votes B, C, D invalid (1 vs 3 — minority by count)
  4. DetermineValidDealersWithConsensus marks all dealers invalid
  5. ComputeGroupPublicKey returns "no valid dealers found"
  6. CompleteDKG returns an error → DKG permanently stuck for this epoch

B, C, D together hold only ~40% of slots and could never form a DKG quorum alone — yet they can destroy the epoch's DKG by voting as a participant-count majority.

Root cause

DetermineValidDealersWithConsensus counts one vote per participant regardless of slot weight:

for _, verification := range epochBLSData.VerificationSubmissions {
    if verification != nil && len(verification.DealerValidity) > 0 {
        totalVotes++
        if verification.DealerValidity[dealerIndex] {
            validVotes++
        }
    }
}
dealerIsValid := totalVotes > 0 && validVotes > totalVotes/2

While everywhere else in the DKG pipeline thresholds are measured in slots, not participant count.

Impact

High (DoS) — a minority coalition of participants (by slot weight) can permanently break DKG for an epoch by voting down dealers that collectively hold a slot majority. This blocks threshold signing for the entire epoch.

Fix Direction

Replace the unweighted participant vote count with a slot-weighted vote in DetermineValidDealersWithConsensus, consistent with how quorum is measured everywhere else:

validSlots := uint32(0)
totalSlots := uint32(0)

for i, verification := range epochBLSData.VerificationSubmissions {
    if verification != nil && len(verification.DealerValidity) > 0 {
        participant := epochBLSData.Participants[i]
        slots := participant.SlotEndIndex - participant.SlotStartIndex + 1
        totalSlots += slots
        if dealerIndex < len(verification.DealerValidity) && verification.DealerValidity[dealerIndex] {
            validSlots += slots
        }
    }
}

dealerIsValid := totalSlots > 0 && validSlots > totalSlots/2

💬 Comments (2)

@Mayveskii commented 2026-03-03 12:56 UTC

Fix submitted in PR #852

@x0152 commented 2026-03-12 20:08 UTC

Hi @Mayveskii

As discussed in #852, this problem is already covered by issue #823 Could you please close this issue?

Thanks!

P.S. I think #848 could be closed as well (as we discussed in #851)

🔄 Auto-synced from Issue #849 every hour.