[Resource Topic] 2021/1375: How to Prove Schnorr Assuming Schnorr: Security of Multi- and Threshold Signatures

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Title:
How to Prove Schnorr Assuming Schnorr: Security of Multi- and Threshold Signatures

Authors: Elizabeth Crites, Chelsea Komlo, Mary Maller

Abstract:

In this paper, we present new techniques for proving the security of multi- and threshold signature schemes under discrete logarithm assumptions in the random oracle model. The purpose is to provide a simple framework for analyzing the relatively complex interactions of these schemes in a concurrent model, thereby reducing the risk of attacks. We make use of proofs of possession and prove that a Schnorr signature suffices as a proof of possession in the algebraic group model without any tightness loss. We introduce and prove the security of a simple, three-round multisignature \mathsf{SimpleMuSig}. Using our new techniques, we prove the concurrent security of a variant of the \mathsf{MuSig2} multisignature scheme that includes proofs of possession as well as the \mathsf{FROST} threshold signature scheme. These are currently the most efficient schemes in the literature for generating Schnorr signatures in a multiparty setting. Our variant of \mathsf{MuSig2}, which we call \mathsf{SpeedyMuSig}, has faster key aggregation due to the proofs of possession.

ePrint: https://eprint.iacr.org/2021/1375

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