Schnorr Signature

A Schnorr signature is a digital signature scheme, specified for bitcoin in BIP 340 and activated with the Taproot upgrade in November 2021, that produces fixed 64-byte signatures with cleaner mathematical properties than the ECDSA scheme bitcoin launched with. Its patent, which delayed adoption industry-wide, expired in 2008, the same year bitcoin was announced.

Why it matters

Schnorr's key property is linearity: signatures and keys can be added together meaningfully. That allows multiple signers to produce a single signature under a single combined key, so a two-of-two multisignature spend, a Lightning channel close, or a plain single-key payment all look identical on-chain. The result is better privacy, smaller transactions, and lower fees for complex arrangements, and it opens the door to protocols such as MuSig2 key aggregation and the adaptor signatures behind proposed point time locked contracts.

How it works

The signer commits to a random nonce point, derives a challenge by hashing the nonce, public key, and message, and publishes the nonce plus a single number combining nonce, challenge, and private key. Verification checks one curve equation. Unlike ECDSA, the construction involves no modular inversion, permits batch verification of many signatures at once, and carries a straightforward security proof. Because the scheme lives on the same secp256k1 curve, existing private keys work unchanged under Taproot addresses.