bls-aggregation-audit

Testing & Quality

L1 trigger - audits BLS signature aggregation: subgroup check, rogue-key attack defense, aggregation order, signing-domain separation.

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Injectable Skill: BLS Aggregation Audit

L1 trigger: L1_PATTERN=true AND (bls/ OR blst OR milagro OR pairing OR aggregate_signature OR proof_of_possession detected in recon subsystem map) Inject Into: depth-consensus-invariant or depth-external Language: Go, Rust, occasionally C Finding prefix: [BLS-N] Status: v0.1 draft, Round 4 exemplars pending

Orchestrator Decomposition Guide

Sections 1, 2: depth-consensus-invariant (protocol-level BLS use)
Section 3: depth-external (library interface, FFI)
Section 4: depth-edge-case (boundary inputs)

When This Skill Activates

Recon identifies BLS12-381 signature use, typically in: Ethereum beacon chain, Filecoin, Celo, Chia, Aptos, or any aggregated-signature consensus. BLS is subtle: textbook descriptions assume defenses (subgroup check, rogue-key defense) that implementations often forget.

1. Library Fingerprinting

Identify the BLS library:

Library	Language	Canonical use
`blst`	C with Go/Rust bindings	Lighthouse, Prysm, Teku
`milagro-crypto`	C	Older clients
`arkworks`	Rust	Aptos, some academic
`celo-bls-zexe`	Rust	Celo
`py_ecc`	Python	Reference implementations
hand-rolled pairing	Various	Red flag — usually wrong

A hand-rolled pairing implementation is a near-certain bug source. Flag it for intensive review.

2. Subgroup Check

BLS signatures live in a subgroup of an elliptic curve group. An attacker can provide a curve point that is valid on the curve but outside the subgroup, leading to signature verification forgeries.

Check:

Is every received signature / public key checked for subgroup membership before use?
In blst, the function is blst_p1_in_g1 / blst_p2_in_g2
In arkworks, there are is_in_correct_subgroup_assuming_on_curve helpers
Protocol-level: when a signature is received from a peer, is subgroup-checked BEFORE aggregation with trusted signatures?
Aggregation without subgroup check: combining a valid sig with a subgroup-invalid sig can corrupt the aggregate

Tag: [BLS-SUBGROUP:{loc}:{checked}]

Known exemplar class: the blst library had a historical subgroup-check advisory; check upgrade logs.

3. Rogue-Key Attack (Proof of Possession)

Rogue-key attack: attacker registers a public key that is the difference of a target's key and the attacker's, then signs a message that verifies as signed by both.

Defenses:

Proof of Possession (PoP): requires each public key to come with a self-signature of the key. Ethereum beacon chain uses this.
Hash-to-point including signer: if the message hash includes the signer's public key, rogue-key is defeated

Check:

For every public key registration / deposit path, is PoP verified?
Is PoP using a distinct domain separator from normal signatures? (Required — otherwise PoP can be forged from a normal sig.)

Tag: [BLS-ROGUE-KEY:{defense}:{status}]

4. Signing Domain Separation

BLS signatures must include a domain tag in the hash-to-point step. This prevents a signature from one protocol being replayed in another.

Check:

List all message types signed (attestation, proposal, slashing, sync committee, PoP, ...)
Each must have a distinct domain tag
Domain tags must be hardcoded constants, not computed from user input
Domain tags must be consistent across client implementations

Tag: [BLS-DOMAIN:{message-type}:{tag}]

5. Aggregation Correctness

5a. Aggregation algebra

Sum of signatures = signature of sum (for BLS). Implementation must use curve addition correctly.
Aggregation of zero signatures is the identity element. Is this handled? Some libraries crash.
Aggregation of signatures on different messages requires a different verification formula (individual verification, not batch).

5b. Order independence

Signature aggregation must be commutative. If the verification result depends on insertion order, that's a bug (and a non-determinism source).

5c. Duplicate handling

If the same signature is aggregated twice, the result doubles. Is this intended? In beacon chain, double-counting an attestation is a slashing condition.

Check: Does the aggregation code deduplicate? Does it track which validators have already contributed?

Tag: [BLS-AGG:{issue}]

6. Hash-to-Curve

The hash-to-curve step (RFC 9380) has multiple valid implementations with subtle differences. Inconsistency across clients is a consensus bug.

Check:

Which hash-to-curve variant is used? SSWU? Icart?
Is the DST (domain separation tag) exactly as specified in the protocol?
Is cofactor clearing applied?

7. FFI Safety

Most BLS libraries are C (blst). Calling C from Go or Rust via FFI is a common bug source:

Check:

Every unsafe block around blst calls: is the pointer valid? Length correct?
Are buffers properly sized for blst's output?
Are errors from blst propagated, or silently ignored?
Is the calling convention correct? (blst's go bindings vs raw cgo)

Tag: [BLS-FFI:{issue}]

8. Boundary conditions

State	Test	Expected
Zero signature	identity-like bytes	spec-defined rejection
Zero public key	identity-like bytes	rejected
Subgroup-invalid sig	on-curve but wrong subgroup	rejected
Empty aggregation	aggregate of zero sigs	identity or error
Duplicate in aggregate	same sig twice	spec-defined
Max validator aggregation	aggregate of all validators	no integer overflow in count
Domain tag swap	attestation sig used as proposal sig	rejected

9. Output schema

Layer: crypto
Bug class: subgroup / rogue-key / domain-sep / aggregation / hash-to-curve / ffi-safety
Preferred evidence tags: [CONFORMANCE-PASS] (test vectors) > [DIFF-PASS] (against reference) > [LSP-TRACE]
Severity baseline: Critical for subgroup/rogue-key; High for domain-sep; Medium for aggregation boundary

10. Known bug exemplars (v0.2 — Round 4 verified)

Eth2 subgroup-check requirement (milagro_bls / blst) — pairing operation is undefined on points outside the prime-order subgroup of G1/G2. Omitting subgroup check breaks strong unforgeability. Eth2 specs explicitly mandate the check for signatures during verification and public keys during deserialization — because early implementations routinely forgot. eth2book BLS chapter; milagro_bls. Skill catch point: Section 2 — the #1 real-world BLS bug class.
BLS rogue-key attack (class-wide) — if attacker publishes pk_attack = pk_hash_of_honest_sigs − pk_honest, they can unilaterally produce valid "aggregate" signatures without cooperation. Mitigated by Proof-of-Possession at registration OR by message-augmentation (MsgAug scheme from BLS IETF draft). Rogue Key Attack writeup; IETF BLS draft. Skill catch point: Section 3 — the #2 real-world BLS bug class.
Aumasson Eth2 beacon client security review (35+ issues) — comprehensive security review of early beacon chain implementations. Found multiple subgroup check omissions, domain separation misconfigurations, and aggregation-order assumptions. Security Review of Ethereum Beacon Clients. Skill catch point: methodology is lifted from this review — the skill is essentially a distillation of the review's checklist.
Avalanche RFC6979 nonce-determinism disputed report (coingeek, 2022) — ava-labs/avalanchego used a Decred library for RFC6979 deterministic signing; researcher James Edwards reported private-key-forgery concerns (disputed by Ava Labs). Whether or not the specific claim holds, the class is real. coingeek report. Skill catch point: Section 7 (FFI safety) + dependency-audit — audit the full signing-library dependency chain, check CVE history, verify deterministic-nonce generation doesn't leak secret bits.

Methodology nuance from Round 4 (subgroup + rogue-key are the #1 and #2 real classes)

Reorder Sections 2 and 3 to reflect priority: subgroup check omission and rogue-key are the two most common real-world BLS failures. Every BLS verifier audit should START with these two checks, not end with them.

11. Fallback if primitives unavailable

Find BLS library import: import "github.com/supranational/blst" or similar
Read every call site
Check every signature-verification function for subgroup check presence
Grep for DOMAIN_SEPARATION, DOMAIN_, DST

Cross-references

Related: consensus-safety-invariants (fork-choice uses aggregated attestations), light-client-proof-verification (validator set signing)
Consumed by: depth-consensus-invariant, depth-external
Severity: docs/l1-mode/severity-matrix.md