In blockchains such as Bitcoin and Ethereum, transactions represent the primary mechanism that the external world can use to trigger a change of blockchain state. Transactions serve as key sources of evidence and play a vital role in forensic analysis. Timed transaction refers to a specific class of service that enables a user to schedule a transaction to change the blockchain state during a chosen future time-frame. This paper proposes T-Watch, a decentralized and cost-efficient approach for users to schedule timed execution of any type of transaction in Ethereum with privacy guarantees. T-Watch employs a novel combination of threshold secret sharing and decentralized smart contracts. To protect the private elements of a scheduled transaction from getting disclosed before the future time-frame, T-Watch maintains shares of the decryption key of the scheduled transaction using a group of executors recruited in a blockchain network before the specified future time-frame and restores the scheduled transaction at a proxy smart contract to trigger the change of blockchain state at the required time-frame. To reduce the cost of smart contract execution in T-Watch, we carefully design the proposed protocol to run in an optimistic mode by default and then switch to a pessimistic mode once misbehaviors occur. Furthermore, the protocol supports users to form service request pooling to further reduce the gas cost. We rigorously analyze the security of T-Watch and implement the protocol over the Ethereum official test network. The results demonstrate that T-Watch is more scalable compared to the state of the art and could reduce the cost by over 90% through pooling.

翻译：在比特币和以太坊等区块链中，交易是外部世界触发区块链状态变更的主要机制，也是取证分析的关键证据来源。定时交易是一类特殊服务，允许用户在选定的未来时间范围内调度交易以改变区块链状态。本文提出T-Watch——一种去中心化且经济高效的方法，使用户能够在以太坊中定时执行任意类型交易，同时保障隐私。T-Watch创新性地结合了门限秘密共享与去中心化智能合约。为保护调度交易的私有元素在未来时间范围前不被泄露，T-Watch在指定未来时间范围前，通过区块链网络中招募的一组执行器维护调度交易解密密钥的份额，并在代理智能合约中恢复调度交易，从而在所需时间触发区块链状态变更。为降低T-Watch中智能合约执行成本，我们精心设计协议以默认运行乐观模式，并在出现异常行为时切换至悲观模式。此外，该协议支持用户形成服务请求池以进一步降低燃料成本。我们对T-Watch的安全性进行了严格分析，并在以太坊官方测试网络上实现了该协议。结果表明，与现有技术相比，T-Watch具有更优的可扩展性，且通过池化机制可降低超过90%的成本。