MuSig1 - A Reasonably Secure Multisig Scheme

MuSig1 is a 3-round multisignature protocol built on Schnorr signatures that allows co-signers to cooperatively sign a message without trusting each other or proving Knowledge-of-Secret-Key. The post walks through the security problems that must be solved: Rogue Key Attacks (addressed via per-signer key coefficients derived from namespaced hashes) and Wagner's Generalized Birthday Attack (addressed via nonce commitments). The full MuSig1 protocol is then presented step-by-step: key aggregation with key coefficients, nonce commitments, message fixation before nonce reveal, partial signing, and final signature aggregation and verification.