# MuSig2 ## Metadata **Status**:: #x **Zettel**:: #zettel/fleeting **Created**:: [[2025-11-06]] **Reference**:: [[Nick et al. - MuSig2 Simple Two-Round Schnorr Multi-Signatures]] ## Algorithm ![[MuSig2.png]] ## MuSig2* $ \mathrm{KeyAggCoef}^*(L, X) = \begin{cases} 1 & \text{if } \mathrm{IsSecond}(L, X) \\ H_{\mathrm{agg}}(L, X) & \text{otherwise.} \end{cases} $ The paper defines the predicate $\mathrm{IsSecond}(L, X)$ to be true if and only if X is the smallest element in L such that $X \ne min(L)$. An alternative is to let the second key be $X_i$, where $i$ is the smallest index such that $X_i \ne X_1$. Ensure that the coefficient is not 1 if there's only one key and its copies in the public keys set.