Blockchain Governance via Sharp Anonymous Multisignatures

Electronic voting has occupied a large part of the cryptographic protocols literature. The recent reality of blockchains---in particular their need for online governance mechanisms---has put new parameters and requirements to the problem. We identify the key requirements of a blockchain governance mechanism, namely correctness (including eliminative double votes), voter anonymity, and traceability, and investigate mechanisms that can achieve them with minimal interaction and under assumptions that fit the blockchain setting. First, we define a signature-like primitive, which we term sharp anonymous multisignatures (in short, #AMS) that tightly meets the needs of blockchain governance. In a nutshell, #AMSs allow any set of parties to generate a signature, e.g., on a proposal to be voted-upon, which if posted on the blockchain hides the identities of the signers/voters, but reveals their number. This can be seen as a (strict) generalization of threshold ring signatures (TRS). We next turn to constructing such #AMSs and using them in various governance scenarios---e.g., single vs. multiple vote per voter. To this direction, we observe that although the definition of TRS does not imply #AMS, one can compile some of the existing TRS constructions into #AMS. This raises the question: What is the TRS structure that allows such a compilation? To answer the above, we devise templates for TRSs. Our templates encapsulate and abstract the structure that allows for the above compilation---most of the TRS schemes that can be compiled into #AMS are, in fact, instantiations of our template. This abstraction makes our template generic for instantiating TRSs and #AMSs from different cryptographic assumptions (e.g., DDH, LWE, etc). One of our templates is based on chameleon hashing and we explore a framework of lossy chameleon hashes to fully understand its nature. Finally, we turn to how #AMS schemes can be used in our applications. We provide fast (in some cases non-interactive) #AMS-based blockchain governance mechanisms for a wide spectrum of assumptions on the honesty (semi-honest vs malicious) and availability of voters and proposers