Collusion-tolerant consensus

Collusion-tolerant consensus


Incentive-compatible consensus is hard

When describing the state model, we imagined Themelio as if it were a magic oracle that always correctly maintains the world state and applies updates to it every block. In the real world, though, we require a procedure to ensure both consistency – all Themelio users observe the same world state at every block height — and validity — the world state can only change through valid transactions. In fact, consensus is ultimately what drives the all-important endogenous trust of blockchains.

Unfortunately, consensus is also a source of major, headache-inducing problems. Contentious blockchain governance problems are often tied to consensus-related issues such as decentralization (for example, the controversy surrounding Bitcoin block sizes), and attacks on existing blockchains, such as the 51% attack on Namecoin, selfish mining attacks on Bitcoin, etc, focus on exploiting problems in consensus.

An important fact to note is that blockchain consensus is unlike consensus in other distributed systems. Because it aims at endogenous trust, it must rigorously model incentives, rather than simply making assumptions about fault tolerance. To truly achieve incentive-compatible endogenous trust, we cannot rely on the typical approach of considering ideal “honest” behavior and then positing an “adversary” with certain powers.

Yet cryptoeconomic mechanism design is a relatively young field full of uncertainties — game-theoretical models often give results different from actual empirical observations — and creating a consensus mechanism that is incentive-compatible under a wide variety of real-world conditions has proven to be perniciously difficult.

Simulating a monopoly with Synkletos

With its Synkletos consensus game, the details of which are available as a design whitepaper and a concrete specification, Themelio takes a unique, “economics-first” approach to designing consensus and related cryptoeconomic mechanisms. We start by introducing a very pessimistic model: a despotic blockchain totally controlled by one rational profit-maximizing entity. We consider what sort of blockchain rules would such a despot enforce, in the absence of external attackers. Perhaps surprisingly, we find that a rational monopoly would in fact behave in a trustworthy manner — the problem with centralized systems that leads to trust failures is generally “irrationality”, not monopoly.

This insight lets us design Themelio’s consensus game. In Synkletos’ design whitepaper , we show that a variant of proof-of-stake with a few critical departures from existing designs elegantly incentivizes a large, permissionless group of stakeholders to simulate this ideal rational monopoly as a whole, regardless of whether they coordinate their actions or not. The most notable departure is the presence of a “fee cartel” influenced by EIP-1551, where the protocol charges a minimum fee for transactions as a part of the transaction validation rules, and this minimum fee is sequentially voted upon by stakeholders, converging to the median of their votes. Such a fee cartel also eliminates the well-known incentive incompatibility of a blockchain funded entirely through auction-based transaction fees, as well as stabilizing fee levels.