Mixed-Weight Committee Selection in Proof-of-Stake: Tunable Stake-Baseline Mixing with Exponential Tail Guarantees and Incentive Compatibility

Abstract

Proof-of-Stake (PoS) blockchains often select committees in direct proportion to stake, which makes security sensitive to large validators and stake concentration. In such settings, a purely stake-based lottery can sometimes produce committees whose adversarial share crosses the safety threshold, even if the global adversarial stake remains below one third. This paper introduces a simple mixed-weight rule that combines stake with a bounded baseline distribution through a single mixing parameter λ. The rule leaves committee size, rewards, and VRF-based sortition unchanged, but pulls weight away from highly concentrated positions. Proved that, whenever the adversary is more concentrated than the baseline, the expected adversarial seats fall linearly in λ, while standard concentration bounds show an exponential drop in committee-capture probability. While the mechanism relies on entity-level attribution (or high-cost identities) to prevent Sybil attacks, experiments on ten production PoS networks indicate that modest mixing (around λ=0.3) reduces expected adversarial seats by about one quarter and tightens worst-case guarantees by orders of magnitude.