T-Watch: Towards Timed Execution of Private Transaction in Blockchains

TL;DR

T-Watch tackles the challenge of privately scheduling and executing timed transactions in Ethereum without relying on a single trusted party. It combines threshold secret sharing with a committee-driven smart-contract architecture to protect private transaction data until a prescribed time, while offering three execution paths (T-Opt, T-Pes, T-Pool) to balance cost and security. The protocol is analyzed under rational-adversary threat models ($\\mathcal{A}$-threshold and $\\mathcal{A}$-budget), and implemented on the Ethereum testnet, demonstrating substantial gas savings (over 90%) via pooling and favorable scalability against existing approaches. The work introduces mechanisms such as punishment deposits, reputation-weighted remuneration, and contract-splitting to ensure honest participation, and shows that T-Watch can extend to other EVM-compatible blockchains, enabling private, timed interactions across diverse applications.

Abstract

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.

Figures (8)

• Figure 1: EOA-only pattern (a) and EOA-CA pattern (b) to update the state of blockchain
• Figure 2: The user-driven architecture (a), the executor-driven architecture (b), and the committee-driven architecture (c) for sending timed transactions. The solid/dotted lines indicate on-chain/off-chain operations, respectively.
• Figure 3: A flowchart of the three execution paths of T-Watch.
• Figure 4: An overview of the proposed T-Watch protocol.
• Figure 5: The protocol that implements T-Watch. A step with a gray bullet (e.g., $\colorbox{gray!28}{3}$) refers to an off-chain operation not recorded by blockchain while a step with a white bullet (e.g., 2) refers to an on-chain operation recorded by blockchain.

Theorems & Definitions (10)

• Definition 1
• Definition 2
• Theorem 1
• Lemma 1
• proof
• Theorem 2
• Lemma 2
• proof
• Lemma 3
• proof