Nakamoto Consensus from Multiple Resources

TL;DR

This paper classifies secure weight functions for longest-chain blockchain protocols that incorporate multiple resource types, providing a theoretical foundation for designing more robust and diverse blockchain consensus mechanisms.

Contribution

It offers a complete classification of secure weight functions in an idealized model, enabling the design of new longest-chain blockchain protocols with multiple resources.

Findings

Homogeneity of degree one in resources V and W ensures security.

Includes Bitcoin's W=W and Chia's SV as special cases.

Proposes alternative secure resource combinations like sqrt(W1)*sqrt(W2).

Abstract

The blocks in the Bitcoin blockchain record the amount of work W that went into creating them through proofs of work. When honest parties control a majority of the work, consensus is achieved by picking the chain with the highest recorded weight. Resources other than work have been considered to secure such longest-chain blockchains. In Chia, blocks record the amount of space S (via a proof of space) and sequential computational steps V (via a VDF). In this paper, we ask what weight functions {\Gamma}(S,V,W) (that assign a weight to a block as a function of the recorded space, speed, and work) are secure in the sense that whenever the weight of the resources controlled by honest parties is larger than the weight of adversarial parties, the blockchain is secure against private double-spending attacks. We completely classify such functions in an idealized "continuous" model: {\Gamma}(S,V,W) is secure against private double-spending attacks if and only if it is homogeneous of degree one in the timed resources V and W, i.e., {\alpha}{\Gamma}(S,V,W)={\Gamma}(S,{\alpha}V, {\alpha}W). This includes Bitcoin rule {\Gamma}(S,V,W)=W and Chia rule {\Gamma}(S,V,W) = SV. In a more realistic model where blocks are created at discrete time-points, one additionally needs some mild assumptions on the dependency on S (basically, the weight should not grow too much if S is slightly increased, say linear as in Chia). Our classification is more general and allows various instantiations of the same resource. It provides a powerful tool for designing new longest-chain blockchains. E.g., consider combining different PoWs to counter centralization, say the Bitcoin PoW W_1 and a memory-hard PoW W_2. Previous work suggested to use W_1+W_2 as weight. Our results show that using {\sqrt}(W_1){\cdot}{\sqrt}(W_2), {\min}{W_1,W_2} are also secure, and we argue that in practice these are much better choices.