TYPE 0x0004 · SIGNATURE FAMILY

MUSIG_THRESHOLD

MuSig2/FROST aggregate threshold signature verification. M-of-N signers produce a single aggregate Schnorr signature off-chain. On-chain footprint is identical to single-sig (~131 bytes) regardless of M or N.

Signature Non-Invertible

Ladder Diagram

Fields

Field	Data Type	Size	Side	Description
threshold_m	NUMERIC	1-4 B	Conditions	Minimum number of signers required (M)
group_size_n	NUMERIC	1-4 B	Conditions	Total number of signers in the group (N)
pubkey	PUBKEY	32-33 B	Witness	Aggregate public key (verified via Merkle leaf)
signature	SIGNATURE	64-65 B	Witness	Aggregate Schnorr signature (single sig regardless of M/N)

Public key folded into Merkle leaf via merkle_pub_key (PubkeyCountForBlock = 1). No key field in conditions.

Wire Format Breakdown

Conditions side (in scriptPubKey):

0x0004 0 3 NUMERIC M NUMERIC N = 10 bytes

Witness side (in input witness):

0x0004 0 2 PUBKEY · 32B SIGNATURE · 64B = 104 bytes

Total (conditions + witness) ~131 bytes

Compared to MULTISIG (3-of-5): 549 bytes → 131 bytes (saves 76%). On-chain, MUSIG_THRESHOLD is indistinguishable from single-sig.

Evaluation Logic

1. Validate M ≥ 1, N ≥ M, N ≤ 100. If invalid → ERROR

2. Verify witness PUBKEY against Merkle leaf (merkle_pub_key). If mismatch → UNSATISFIED

3. Enforce Schnorr-only: SCHEME must be 0x01 (SCHNORR) or absent. No PQ path. If invalid → ERROR

4. Verify aggregate SIGNATURE against PUBKEY using BIP-340 Schnorr. If invalid → UNSATISFIED

5. Verification passed → SATISFIED

Return Values

Condition	Result
Missing required fields	ERROR
M = 0 or M > N or N > 100	ERROR
PQ signature scheme requested	ERROR
Merkle leaf verification fails	UNSATISFIED
Schnorr signature verification fails	UNSATISFIED
Aggregate key matches commitment AND signature valid	SATISFIED

JSON Wire Format

Conditions (scriptPubKey)

{
  "type": "MUSIG_THRESHOLD",
  "inverted": false,
  "fields": [
    { "type": "PUBKEY", "hex": "02abc1...33 bytes" },
    { "type": "NUMERIC", "value": 2 },
    { "type": "NUMERIC", "value": 3 }
  ]
}

Witness (input)

{
  "type": "MUSIG_THRESHOLD",
  "inverted": false,
  "fields": [
    { "type": "PUBKEY", "hex": "02abc1...33 bytes" },
    { "type": "SIGNATURE", "hex": "e4f2a1...64 bytes" }
  ]
}

Use Cases

Privacy-Preserving Multisig

A 3-of-5 corporate treasury uses MUSIG_THRESHOLD. On-chain, the spend looks identical to a single-sig P2TR output — no observer can distinguish it from a solo wallet or determine the threshold policy.

Lightning Channel Opens

2-of-2 funding outputs using MUSIG_THRESHOLD are indistinguishable from regular P2TR spends, improving the anonymity set for all channel participants.

Fee-Efficient DAOs

A 7-of-11 governance multisig pays the same fee as a single-sig transaction. The M and N parameters are committed in conditions for policy enforcement, but the on-chain signature is always 64 bytes.

← ADAPTOR_SIG KEY_REF_SIG →