[Paper Review] Towards Blockchain-enabled Searchable Encryption

Qiang Tang|arXiv (Cornell University)|Aug 26, 2019

Blockchain Technology Applications and Security15 references4 citations

TL;DR

This paper proposes two blockchain-enabled frameworks for symmetric searchable encryption that preserve the original scheme's privacy while achieving fairness in search operations. By using blockchain only for storing signatures, encrypted results, and public metadata—without exposing trapdoors or indices—it ensures verifiable correctness and rewards for servers without increasing privacy leakage beyond the original scheme.

ABSTRACT

Distributed Leger Technologies (DLTs), most notably Blockchain technologies, bring decentralised platforms that eliminate a single trusted third party and avoid the notorious single point of failure vulnerability. Since Nakamoto's Bitcoin cryptocurrency system, an enormous number of decentralised applications have been proposed on top of these technologies, aiming at more transparency and trustworthiness than their traditional counterparts. These applications spread over a lot of areas, e.g. financial services, healthcare, transportation, supply chain management, and cloud computing. While Blockchain brings transparency and decentralised trust intuitively due to the consensus of a (very large) group of nodes (or, miners), it introduces very subtle implications for other desirable properties such as privacy. In this work, we demonstrate these subtle implications for Blockchain-based searchable encryption solutions, which are one specific use case of cloud computing services. These solutions rely on Blockchain to achieve both the standard privacy property and the new fairness property, which requires that search operations are carried out faithfully and are rewarded accordingly. We show that directly replacing the server in an existing searchable encryption solution with a Blockchain will cause undesirable operational cost, privacy loss, and security vulnerabilities. The analysis results indicate that a dedicated server is still needed to achieve the desired privacy guarantee. To this end, we propose two frameworks which can be instantiated based on most existing searchable encryption schemes. Through analysing these two frameworks, we affirmatively show that a carefully engineered Blockchain-based solution can achieve the desired fairness property while preserving the privacy guarantee of the original searchable encryption scheme simultaneously.

Motivation & Objective

• To address the lack of fairness and trust in cloud-based searchable encryption, where servers may not perform searches faithfully.
• To analyze the risks of naively replacing cloud servers with blockchain in searchable encryption systems.
• To design blockchain-integrated frameworks that maintain the privacy guarantees of existing symmetric searchable encryption schemes.
• To ensure that search operations are verifiable and that honest servers are fairly compensated.
• To demonstrate that blockchain can be used effectively in searchable encryption without introducing new privacy or security vulnerabilities.

Proposed method

• Introduces a modified setup phase that generates virtual identifiers using HMAC for each keyword, binding file identifiers to search results.
• Uses blockchain solely to store public information: encrypted search results, signatures, public keys, and deposits.
• Employs a dispute resolution mechanism where the server is punished if it fails to return correct results, verified via HMAC-based integrity checks.
• Relies on standard cryptographic primitives: EU-CMA secure signatures and IND-CPA secure encryption, ensuring confidentiality under unilateral integrity assumptions.
• Designs a retrieval and validation phase where clients verify correctness using the HMAC-protected virtual identifiers.
• Ensures that only minimal data—signatures, encrypted results, and public keys—is exposed on the blockchain, preserving privacy.

Experimental results

Research questions

• RQ1 Can blockchain be used to enforce fairness in searchable encryption without compromising the privacy of the original scheme?
• RQ2 What are the privacy and security implications of directly replacing a server with a blockchain in searchable encryption systems?
• RQ3 How can blockchain be integrated in a way that minimizes overhead while enabling verifiable search operations?
• RQ4 What cryptographic mechanisms are necessary to ensure both fairness and confidentiality in blockchain-enhanced searchable encryption?
• RQ5 Can a hybrid model with a dedicated server and blockchain support achieve better privacy and fairness than fully decentralized alternatives?

Key findings

• Directly replacing a server with a blockchain in searchable encryption leads to increased operational costs, privacy leakage, and security vulnerabilities.
• The proposed frameworks preserve the original scheme’s privacy guarantees because the blockchain only stores public metadata, not trapdoors or encrypted indices.
• The blockchain is used only to store signatures, encrypted results, and public keys, ensuring no additional information is leaked to the server.
• Fairness is achieved through a dispute resolution mechanism where misbehaving servers are penalized, and honest servers are rewarded based on verifiable results.
• The overhead of blockchain involvement is minimal in normal operation, as only a few public records are written per search.
• The solution maintains IND-CPA confidentiality and EU-CMA unforgeability, ensuring security under standard assumptions.