Towards Searchable and Verifjable Blockchain Cheng Xu Ce Zhang April 8, 2019 Department of Computer Science Hong Kong Baptist University
Background • Blockchain is a distributed ledger maintained by a community of (untrusted) users • Decentralization • Consensus • Immutability • Provenance • A wide range of applications • Record Keeping • Smart Contracts • 1/17 • Blockchain � = Bitcoin
Background • Blockchain is a distributed ledger maintained by a community of (untrusted) users • Decentralization • Consensus • Immutability • Provenance • A wide range of applications • Record Keeping • Smart Contracts • Fig. 1: Blockchain Structure [Credit: Wikipedia] 1/17 • Blockchain � = Bitcoin
Background • Blockchain is a distributed ledger maintained by a community of (untrusted) users • Decentralization • Consensus • Immutability • Provenance • A wide range of applications • Record Keeping • Smart Contracts Fig. 2: Blockchain Applications [Credit: FAHM Technology Partners] 1/17 • Blockchain � = Bitcoin • · · ·
• Issue : relying on a trusted party who can faithfully execute user queries Blockchain Database Solutions • Increasing demand to search the data stored in blockchains • Blockchain database solutions to support SQL-like queries Fig. 3: Blockchain Database Solutions 2/17 SwarmDB
Blockchain Database Solutions • Increasing demand to search the data stored in blockchains • Blockchain database solutions to support SQL-like queries Fig. 3: Blockchain Database Solutions • Issue : relying on a trusted party who can faithfully execute user queries 2/17 SwarmDB
• Alternative approach : becoming light node and outsource computation R R Q Q Blockchain Search Problem Light Node • Question : how to ensure integrity? Miner Full Node Miner Full Node • Integrity assurance : query results retrieved from the blockchain should be publicly • Low cost: maintaining block headers only (< 50 MB for Bitcoin) • Computation: to verify the consensus proofs • Storage: to store a complete replicate (240 GB for Bitcoin as of Mar 2019) • High cost verifjable 3/17 • Becoming full node • Network: to synchronize with the network
Blockchain Search Problem • Low cost: maintaining block headers only (< 50 MB for Bitcoin) • Question : how to ensure integrity? Miner Full Node Light Node • Integrity assurance : query results retrieved from the blockchain should be publicly Miner Full Node • Computation: to verify the consensus proofs • Storage: to store a complete replicate (240 GB for Bitcoin as of Mar 2019) • High cost verifjable 3/17 • Becoming full node • Network: to synchronize with the network • Alternative approach : becoming light node and outsource computation R R Q · · · Q
Blockchain Search Problem • Low cost: maintaining block headers only (< 50 MB for Bitcoin) • Question : how to ensure integrity? Miner Full Node Light Node • Integrity assurance : query results retrieved from the blockchain should be publicly Miner Full Node • Computation: to verify the consensus proofs • Storage: to store a complete replicate (240 GB for Bitcoin as of Mar 2019) • High cost verifjable 3/17 • Becoming full node • Network: to synchronize with the network • Alternative approach : becoming light node and outsource computation R R Q · · · Q
Solution #1: Smart Contract VM Smart Contract CPU Computation Blockchain RAM Storage Blockchain • A trusted program to execute user-defjned computation Computer Traditional • Offer trusted storage and computation capabilities • Execution integrity is ensured by the consensus protocol • Smart Contract reads and writes blockchain data upon the blockchain 4/17 • Function as a trusted virtual machine
• Drawbacks • Poor scalability: transaction rate of the blockchain is limited • Privacy concern: query history is permanently and publicly stored in blockchain • High cost: executing smart contract in ETH requires paying gas to miners decentralized, reliable and fair realization,” in IEEE INFOCOM , Honolulu, HI, USA, 2018, pp. 792–800. S. Hu, C. Cai, Q. Wang, C. Wang, X. Luo, and K. Ren, “Searching an encrypted cloud meets blockchain: A 24 USD per query ) 0 18 Ether ( INFOCOM 2018 requires 4 201 232 gas Solution #1: Smart Contract • Leverage Smart Contract for trusted computation • Long latency: long time for consensus protocol to confjrm a block [Credit: Oscar W] • Users read results from blockchain • Users submit query parameters to blockchain 5/17 • Miners execute computation and write results into blockchain
Solution #1: Smart Contract • Leverage Smart Contract for trusted computation • Users submit query parameters to blockchain • Users read results from blockchain [Credit: Oscar W] • Long latency: long time for consensus protocol to confjrm a block • High cost: executing smart contract in ETH requires paying gas to miners S. Hu, C. Cai, Q. Wang, C. Wang, X. Luo, and K. Ren, “Searching an encrypted cloud meets blockchain: A decentralized, reliable and fair realization,” in IEEE INFOCOM , Honolulu, HI, USA, 2018, pp. 792–800. 5/17 • Miners execute computation and write results into blockchain • Drawbacks • Poor scalability: transaction rate of the blockchain is limited • Privacy concern: query history is permanently and publicly stored in blockchain ( INFOCOM 2018 requires 4 201 232 gas = 0 . 18 Ether = 24 USD per query )
• Outsource queries to full node and verify the results using VC • General VC: Expressive but high overhead • Authenticated Data Structure (ADS)-based VC: Effjcient but requiring customized designs Solution #2: Verifjable Computation • Verifjcation Computation (VC) • Computation is outsourced to untrusted service provider • Service provider returns results with cryptographic proof • Users verify integrity of results using the proof 6/17
Solution #2: Verifjable Computation • Verifjcation Computation (VC) • Computation is outsourced to untrusted service provider • Service provider returns results with cryptographic proof • Users verify integrity of results using the proof 6/17 • Outsource queries to full node and verify the results using VC • General VC: Expressive but high overhead • Authenticated Data Structure (ADS)-based VC: Effjcient but requiring customized designs
Our Solutions • vChain: Enabling Verifjable Boolean Range Queries over Blockchain Databases ( SIGMOD 2019 ) brid Storage in Blockchain ( ICDE 2019 ) 7/17 • GEM 2 -Tree: Enabling Gas-Effjcient Authenticated Range Queries for Hy-
vChain: Enabling Verifjable Boolean Range Queries over Blockchain Databases Cheng Xu, Ce Zhang, and Jianliang Xu ACM SIGMOD 2019
R VO R VO Q Q • Soundness: none of the objects returned as results have been tampered with and all of • Completeness: no valid result is missing regarding the query window or subscription period vChain — Problem Defjnition Full Node them satisfy the query conditions • Security requirements Miner Light Node • Problem : integrity-assured search over blockchain data Full Node Miner • • Commercial interest • Security vulnerabilities • Program glitches • Full node not trusted • Queries are outsourced to full nodes • Users become light nodes • System Model : 8/17
• Soundness: none of the objects returned as results have been tampered with and all of • Completeness: no valid result is missing regarding the query window or subscription period vChain — Problem Defjnition Full Node them satisfy the query conditions • Security requirements Miner Full Node • Problem : integrity-assured search over blockchain data Light Node Miner • • Commercial interest • Security vulnerabilities • Program glitches • Full node not trusted • Queries are outsourced to full nodes • System Model : 8/17 • Users become light nodes R R Q · · · Q
• Soundness: none of the objects returned as results have been tampered with and all of • Completeness: no valid result is missing regarding the query window or subscription period vChain — Problem Defjnition Full Node them satisfy the query conditions • Security requirements Miner Full Node • Problem : integrity-assured search over blockchain data Light Node Miner • Commercial interest • Security vulnerabilities • Program glitches • Full node not trusted • Queries are outsourced to full nodes • System Model : 8/17 • Users become light nodes R R Q · · · Q • · · ·
vChain — Problem Defjnition Full Node • Completeness: no valid result is missing regarding the query window or subscription period them satisfy the query conditions • Soundness: none of the objects returned as results have been tampered with and all of • Security requirements Miner Full Node • Problem : integrity-assured search over blockchain data Light Node Miner • Commercial interest • Security vulnerabilities • Program glitches • Full node not trusted • Queries are outsourced to full nodes • System Model : 8/17 • Users become light nodes � R , VO � � R , VO � Q · · · Q • · · ·
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