MuSig

MuSig is a family of multisignature protocols that lets several parties jointly produce one Schnorr signature indistinguishable from a single signer's, with MuSig2 standardized as BIP 327 in 2023. Instead of publishing multiple signatures on chain, participants aggregate their public keys into one key and cooperate to sign, so a 3-of-3 arrangement settles as one ordinary-looking signature.

Why it matters

The payoff is privacy and cost. A MuSig spend reveals nothing about how many parties control the coins, and it pays the fee of a single-signature transaction regardless of participant count. Lightning channel closes, exchange cold storage and collaborative custody can all appear on chain as unremarkable single-key spends since Taproot activated in November 2021. Shared control of gold means shared vaults, split bars or trusted custodians; MuSig makes shared control of bitcoin invisible and nearly free, strengthening the argument that bitcoin's custody arrangements can be simultaneously more flexible and more private than anything physical metal supports.

How it works

Each signer contributes nonces and partial signatures across two communication rounds in MuSig2. Schnorr signatures are linear, so the partial signatures sum to one valid signature under the aggregated key. Defenses against rogue-key and nonce-reuse attacks are built into the protocol, which is why the naive approach of simply adding keys together is unsafe and a formally analyzed scheme is needed.