BIP 327 — MuSig2 for BIP340-compatible Multi-Signatures

BIP 327 (MuSig2 for BIP340-compatible Multi-Signatures) specifies a two-round multi-signature protocol in which several signers cooperatively produce a single Schnorr signature under a single aggregate BIP 340 key. On chain, the result is indistinguishable from a single-signer Taproot spend. Repository status: Deployed.

Debate relevance: Relevant to Lightning (Taproot channels), collaborative custody, and privacy debates — multiparty arrangements that no longer look multiparty on chain. Common misstatements: Conflating MuSig2 key aggregation with script multisig (CHECKMULTISIG/CHECKSIGADD) — MuSig2 yields one key and one signature; script multisig publishes the quorum. Also, "Schnorr aggregates all signatures in a block" — cross-input signature aggregation is not part of any deployed protocol.