Interactive Multi-Sig

Interactive multi-sig is the class of multi-signature schemes where cosigners must actively communicate with each other during signing, rather than signing independently and combining the results offline.

The contrast:

• Non-interactive multisig (classical OP_CHECKMULTISIG, P2SH, P2WSH). Each cosigner signs the transaction independently using their own private key. The script combines the resulting signatures on-chain. Cosigners never need to talk to each other beyond passing the partial transaction around (typically via PSBT).
• Interactive multisig (MuSig2, FROST). Cosigners participate in a multi-round protocol where each contributes randomness and partial signatures that must be combined in a specific cryptographic way to produce a single valid Schnorr signature.

Why interactive is sometimes worth the complexity:

• Aggregated signature on-chain. The output of a MuSig2 or FROST signing session is one 64-byte Schnorr signature that looks identical to a single-sig spend. Significant privacy and fee savings vs. classical N-of-M multisig where N signatures appear on-chain.
• Threshold flexibility. FROST supports m-of-n thresholds where any m signers can produce the signature - no rigid 1-of-1 or n-of-n. Classical multisig is rigid; aggregated thresholds are mathematically richer.

What interactive multisig costs:

• Coordination overhead. Cosigners need to be online simultaneously (or at least in coordinated communication rounds). For multi-time-zone teams or human signers, this is operational friction.
• State management. Aggregated signing protocols are stateful. Cosigners must track nonces across rounds carefully. State loss can break the signing session or, in the worst case, leak keys.
• Higher implementation complexity. Classical multisig is mature and battle-tested. MuSig2 is newer (BIP 327 standardized 2023) and the implementation surface is more sensitive to bugs.

Production status as of 2026:

• Lightning channel construction is increasingly using interactive multisig (2-of-2 MuSig2 for cooperative channel state, instead of classical 2-of-2).
• Federation setups (Fedimint, Liquid functionaries) experiment with FROST.
• Mainstream personal multisig (Sparrow + hardware wallets) still uses classical multisig for operational reasons; non-interactive signing fits human workflow better.

The general direction is toward more aggregated / interactive multisig adoption, but the migration is gradual. Both forms will coexist for years.