Layer 1 (L1) blockchains such as Ethereum are secured under an "honest supermajority of stake" assumption for a large pool of validators who verify each and every transaction on it. This high security comes at a scalability cost which not only effects the throughput of the blockchain but also results in high gas fees for executing transactions on chain. The most successful solution for this problem is provided by optimistic rollups, Layer 2 (L2) blockchains that execute transactions outside L1 but post the transaction data on L1. The security for such L2 chains is argued, informally, under the assumption that a set of nodes will check the transaction data posted on L1 and raise an alarm (a fraud proof) if faulty transactions are detected. However, all current deployments lack a proper incentive mechanism for ensuring that these nodes will do their job ``diligently'', and simply rely on a cursory incentive alignment argument for security. We solve this problem by introducing an incentivized watchtower network designed to serve as the first line of defense for rollups. Our main contribution is a ``Proof of Diligence'' protocol that requires watchtowers to continuously provide a proof that they have verified L2 assertions and get rewarded for the same. Proof of Diligence protocol includes a carefully-designed incentive mechanism that is provably secure when watchtowers are rational actors, under a mild rational independence assumption. Our proposed system is now live on Ethereum testnet. We deployed a watchtower network and implemented Proof of Diligence for multiple optimistic rollups. We extract execution as well as inclusion proofs for transactions as a part of the bounty. Each watchtower has minimal additional computational overhead beyond access to standard L1 and L2 RPC nodes.

翻译：第一层（L1）区块链（如以太坊）的安全依赖于“诚实多数权益”假设，即大量验证者逐一验证链上每笔交易。这种高安全性以可扩展性为代价，不仅影响区块链吞吐量，还导致链上交易执行产生高昂的Gas费用。针对该问题最成功的解决方案是乐观Rollup——第二层（L2）区块链在L1之外执行交易，但将交易数据发布到L1上。此类L2链的安全性非正式地建立在如下假设之上：一组节点会检查L1上发布的交易数据，并在检测到故障交易时发出警报（欺诈证明）。然而，当前所有部署都缺乏适当的激励机制来确保这些节点“勤勉”履行职责，仅依赖粗略的激励一致性论证来保障安全。为解决此问题，我们提出一个带有激励机制的监视器网络，旨在作为Rollup的第一道防线。我们的主要贡献是“勤勉证明”协议，该协议要求监视器持续提供其已验证L2断言的证明，并因此获得奖励。勤勉证明协议包含精心设计的激励机制，在理性行为者假设及温和的理性独立性假设下可证明其安全性。我们提出的系统现已在以太坊测试网上线。我们部署了监视器网络，并为多个乐观Rollup实现了勤勉证明。作为赏金机制的一部分，我们还提取了交易的执行证明和包含证明。每个监视器除访问标准L1和L2 RPC节点外，仅需极少的额外计算开销。