Signature Aggregation

Signature aggregation is the ability to collapse several signatures, or several signers' keys, into one compact Schnorr signature that the network verifies as a unit. It is a direct consequence of the linearity of Schnorr signatures: because the math adds cleanly, the contributions of multiple parties can be summed into a single valid signature for a single combined key. This is the property that makes Taproot multisig so efficient and private.

Key Aggregation with MuSig2

The dominant standard, MuSig2 (BIP327), is an n-of-n scheme that lets a group non-interactively combine their individual public keys into one aggregate key. Spending that key requires a signature cooperatively produced by all participants, exchanged over two communication rounds: nonces first, then partial signatures. The resulting on-chain output is a single BIP340 public key and a single signature, indistinguishable from an ordinary single-signer payment. Observers cannot tell a 1-of-1 from a 5-of-5.

Why It Matters

Aggregation cuts the on-chain weight of multisig dramatically and improves privacy by hiding the participant count and policy. It does carry security trade-offs: MuSig2 is proven secure under a stronger cryptographic assumption than single-signer Schnorr, and its interactive nonce exchange demands disciplined nonce handling to avoid the catastrophe described under nonce reuse.

Aggregation is most often used on the cooperative branch of key path vs script path spend, and the strict nonce hygiene it requires is detailed in nonce reuse.