[Paper Review] SilentDelivery: Practical Timed-delivery of Private Information using Smart Contracts

Chao Li, Balaji Palanisamy|arXiv (Cornell University)|Dec 17, 2019

Blockchain Technology Applications and Security32 references4 citations

TL;DR

SilentDelivery proposes a decentralized, secure, and cost-efficient protocol for timed information delivery using threshold secret sharing and smart contracts on blockchains. It enables private, timely release of sensitive data by distributing decryption key shares among recruited mailmen, ensuring availability and confidentiality while reducing smart contract costs by 85% on Ethereum.

ABSTRACT

This paper proposes SilentDelivery, a secure, scalable and cost-efficient protocol for implementing timed information delivery service in a decentralized blockchain network. SilentDelivery employs a novel combination of threshold secret sharing and decentralized smart contracts. The protocol maintains shares of the decryption key of the private information of an information sender using a group of mailmen recruited in a blockchain network before the specified future time-frame and restores the information to the information recipient at the required time-frame. To tackle the key challenges that limit the security and scalability of the protocol, SilentDelivery incorporates two novel countermeasure strategies. The first strategy, namely silent recruitment, enables a mailman to get recruited by a sender silently without the knowledge of any third party. The second strategy, namely dual-mode execution, makes the protocol run in a lightweight mode by default, where the cost of running smart contracts is significantly reduced. We rigorously analyze the security of SilentDelivery and implement the protocol over the Ethereum official test network. The results demonstrate that SilentDelivery is more secure and scalable compared to the state of the art and reduces the cost of running smart contracts by 85%.

Motivation & Objective

• To address the limitations of centralized timed information delivery services (TIDS), which rely on single points of trust and are vulnerable to breaches and insider attacks.
• To design a decentralized TIDS protocol that ensures privacy, availability, and fairness without relying on trusted third parties.
• To overcome scalability and security challenges in blockchain-based TIDS, particularly regarding visibility of recruitment and high smart contract costs.
• To enable secure, scalable, and low-cost execution of timed delivery using threshold secret sharing and smart contract incentives.
• To provide a practical, production-ready solution for real-world applications requiring private, time-locked information release.

Proposed method

• SilentDelivery uses threshold secret sharing to split the decryption key of private information into shares distributed among a group of n mailmen.
• A sender recruits n mailmen via smart contracts before the target delivery time, with each mailman holding one key share and a copy of the encrypted message.
• The protocol employs 'silent recruitment' to hide recruitment relationships from third parties, preserving privacy until the delivery time.
• It implements 'dual-mode execution'—running in a lightweight mode by default to reduce gas costs, with full verification only when necessary.
• Smart contracts manage the entire process, including pre-locking cryptocurrency deposits as security deposits to penalize misbehavior and ensure fairness.
• The protocol ensures liveness and persistence by leveraging blockchain’s immutability and finality, preventing denial-of-service and denial-of-delivery attacks.

Experimental results

Research questions

• RQ1 How can a decentralized timed information delivery system be designed to ensure privacy, availability, and fairness without relying on trusted third parties?
• RQ2 What mechanisms can prevent third-party monitoring of recruitment relationships in a transparent blockchain environment?
• RQ3 How can the high cost of smart contract execution in blockchain-based TIDS be significantly reduced without compromising security?
• RQ4 What trade-offs exist between security, scalability, and cost efficiency in threshold secret sharing-based timed delivery protocols?
• RQ5 Can a combination of threshold secret sharing and smart contract incentives achieve both strong security and practical deployment on public blockchains like Ethereum?

Key findings

• SilentDelivery reduces the cost of running smart contracts by 85% compared to existing approaches through its dual-mode execution strategy.
• The protocol achieves strong privacy guarantees via silent recruitment, preventing any third party—including the blockchain observer—from learning recruitment relationships before the delivery time.
• By using a group of n mailmen with threshold secret sharing, SilentDelivery ensures message availability as long as at least t (t < n) mailmen remain honest and operational.
• The protocol is formally proven secure under standard cryptographic assumptions, with fairness ensured by smart contract-based security deposits that are confiscated upon misbehavior.
• Evaluation on the Ethereum testnet confirms that SilentDelivery is scalable and practical for real-world deployment, with minimal on-chain footprint during normal operation.
• The integration of threshold secret sharing with smart contracts enables a trustless, decentralized marketplace for timed information delivery, eliminating reliance on centralized servers.