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Security: BLS group key validation falls back to self-validation when previous epoch data is missing #848

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

Location

inference-chain/x/bls/keeper/msg_server_group_validation.go — lines 64–74

Description

When GetEpochBLSData fails with ErrEpochBLSDataNotFound for the previous epoch, the code silently falls back to using the new epoch's own data as the previous epoch:

previousEpochBLSData, err := ms.GetEpochBLSData(ctx, previousEpochId)
if err != nil {
    if errors.Is(err, types.ErrEpochBLSDataNotFound) {
        previousEpochBLSData = newEpochBLSData // fallback to self
    }
}

This creates circular self-validation: - Participants are looked up from previousEpochBLSData — which is now the new epoch's own participants - Slot public keys for per-slot signature verification come from newEpochBLSData.SlotPublicKeys - The final aggregated signature is verified against previousEpochBLSData.GroupPublicKey — which is now the new epoch's own group key

Result: any participant in epoch N can sign and submit a valid group key validation for epoch N using epoch N's own keys — completely bypassing the intended cross-epoch chain-of-custody.

When This Triggers

  • Previous epoch BLS data was pruned (expected over time)
  • State sync or snapshot restore on a new node
  • First epoch after a chain upgrade where old BLS data is absent

Impact

Critical (security) — the cryptographic chain-of-custody for group key transitions is bypassed. A single malicious participant can self-certify a new group key without legitimate cross-epoch consensus.

Fix Direction

Return an error instead of silently falling back:

if errors.Is(err, types.ErrEpochBLSDataNotFound) {
    return nil, fmt.Errorf("previous epoch %d BLS data not found, cannot validate group key", previousEpochId)
}

If bootstrapping for the very first epoch is required, handle it explicitly with a dedicated flag/check rather than a silent fallback.

💬 Comments (2)

@Mayveskii commented 2026-03-03 12:15 UTC

Investigated the fallback path at line 74 of msg_server_group_validation.go.

When previousEpochBLSData is not found, the code assigns previousEpochBLSData = newEpochBLSData. This means: 1. verifyBLSPartialSignatureBlst checks signatures against the new epoch's own slot keys 2. verifyFinalSignatureBlst checks the aggregate against the new epoch's own GroupPublicKey

A validator who controls epoch N's DKG output can trigger this path to get epoch N+1's key accepted without any external verification. Fix: return error when previous epoch data is missing.

PR: https://github.com/Mayveskii/gonka/pull/new/fix/848-bls-self-validation

@Mayveskii commented 2026-03-03 12:16 UTC

Investigated the fallback path at line 74 of msg_server_group_validation.go.

When previousEpochBLSData is not found, the code assigns previousEpochBLSData = newEpochBLSData. This means:

  1. verifyBLSPartialSignatureBlst checks signatures against the new epoch's own slot keys
  2. verifyFinalSignatureBlst checks the aggregate against the new epoch's own GroupPublicKey

A validator who controls epoch N's DKG output can trigger this path to get epoch N+1's key accepted without any external verification. Fix: return error when previous epoch data is missing.

PR: https://github.com/Mayveskii/gonka/pull/new/fix/848-bls-self-validation

Summary

Closes #848

When GetEpochBLSData for previousEpochId returned ErrEpochBLSDataNotFound, the handler silently fell back to previousEpochBLSData = newEpochBLSData.

This allowed any epoch to self-certify its own group key: - partial signatures were verified against the new epoch's own individual keys - the aggregated final signature was verified against the new epoch's own GroupPublicKey

The chain-of-trust between epochs was completely bypassed.

Fix

Removed the fallback entirely. When previous epoch data is unavailable, the handler now returns an explicit error. This is the only correct behavior — group key validation is meaningless without an independent previous epoch as the verifier.

Files changed

  • inference-chain/x/bls/keeper/msg_server_group_validation.go
  • removed unused errors import
  • replaced 13-line silent fallback with a hard error return

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