Towards Rational Consensus in Honest Majority
Varul Srivastava, Sujit Gujar
TL;DR
The paper investigates Atomic Broadcast (ABC) under a generalized rational fault model with $t$ Byzantine and $k$ rational players, introducing four rational types $\theta\in\{0,1,2,3\}$. It proves impossibility results for $\theta\in\{2,3\}$ in partially synchronous/asynchronous settings when $t+k$ reaches critical thresholds, and shows that prior baiting-based RC protocols can admit insecure Nash equilibria in ABC. To address this, it proposes pRFT, a protocol that achieves ABC under $t<\frac{n}{4}$ and $t+k<\frac{n}{2}$ for $\theta=1$, by embedding accountability via Proof-of-Fraud and ensuring Dominant Strategy Incentive Compatibility (DSIC). The design achieves this with $O(n^3)$ message complexity and $O(\kappa n^4)$ message size, aligning with accountability-focused protocols while handling a broader threat model. Overall, the work advances secure ABC in blockchain-like environments by eliminating reliance on baiting equilibria and providing a DSIC, accountable approach under partial synchrony.
Abstract
Distributed consensus protocols reach agreement among $n$ players in the presence of $f$ adversaries; different protocols support different values of $f$. Existing works study this problem for different adversary types (captured by threat models). There are three primary threat models: (i) Crash fault tolerance (CFT), (ii) Byzantine fault tolerance (BFT), and (iii) Rational fault tolerance (RFT), each more general than the previous. Agreement in repeated rounds on both (1) the proposed value in each round and (2) the ordering among agreed-upon values across multiple rounds is called Atomic BroadCast (ABC). ABC is more generalized than consensus and is employed in blockchains. This work studies ABC under the RFT threat model. We consider $t$ byzantine and $k$ rational adversaries among $n$ players. We also study different types of rational players based on their utility towards (1) liveness attack, (2) censorship or (3) disagreement (forking attack). We study the problem of ABC under this general threat model in partially-synchronous networks. We show (1) ABC is impossible for $n/3< (t+k) <n/2$ if rational players prefer liveness or censorship attacks and (2) the consensus protocol proposed by Ranchal-Pedrosa and Gramoli cannot be generalized to solve ABC due to insecure Nash equilibrium (resulting in disagreement). For ABC in partially synchronous network settings, we propose a novel protocol \textsf{pRFT}(practical Rational Fault Tolerance). We show \textsf{pRFT} achieves ABC if (a) rational players prefer only disagreement attacks and (b) $t < \frac{n}{4}$ and $(t + k) < \frac{n}{2}$. In \textsf{pRFT}, we incorporate accountability (capturing deviating players) within the protocol by leveraging honest players. We also show that the message complexity of \textsf{pRFT} is at par with the best consensus protocols that guarantee accountability.