Cryptocurrencies and decentralized platforms are rapidly gaining traction since Nakamoto’s discovery of Bitcoin’s blockchain protocol. These systems use Proof of Work (PoW) to achieve unprecedented security for digital assets. However, the significant power consumption and ecological impact of PoW are leading policymakers to consider stark measures against them and prominent systems to explore alternatives. But these alternatives imply stepping away from key security aspects of PoW.

We present Sprints, a blockchain protocol that achieves almost the same security guarantees as PoW blockchains, but with an order-of-magnitude lower ecological impact, taking into account both power and hardware. To achieve this, Sprints forces miners to mine intermittently. It interleaves Proof of Delay (PoD, e.g., using a Verifiable Delay Function) and PoW, where only the latter bares a significant resource expenditure. We prove that in Sprints the attacker’s success probability is the same as that of legacy PoW. To evaluate practical performance, we analyze the effect of shortened PoW duration, showing a minor reduction in resilience (49% instead of 50%). We confirm the results with a full implementation using 100 patched Bitcoin clients in an emulated network.