Resource-Efficient Mining (REM) with Proofs of Useful Work (PoUW)

Transcription

1 Resource-Efficient Mining (REM) with Proofs of Useful Work (PoUW) Ittay Eyal Technion, EE and IC3 with Fan Zhang, Robert Escriva. Ari Juels, and Robbert van Renesse Workshop on Blockchain Technology and Theory October 16, 2017

2 Fintech Blockchain / DLT Vision Bank to bank transactions (money, securities) Smart contracts infrastructure Security structuring Insurance Provenance (supply chain, art, fair trade) IoT micropayments 2

3 The Blockchain Log Blockchain header block 3

4 The Blockchain Log Blockchain header block 4

5 The Blockchain Log Blockchain hash( ) < target* * target: a deterministic function of previous blocks 5

6 The Blockchain 6

7 The Blockchain 7

8 The Blockchain Block validation: Proof of work Ok one hash function (and transactions Ok) 8

9 PoW Requirements Memoryless random distribution Tunable difficulty O(1) validation All data on the blockchain 9

10 Fork Resolution Longest chain wins Minority attacker cannot out-run honest parties 10

11 Mining Farms 11

12 PoW: Proof of Waste? Block proves (statistically) real-world waste Capital expenditure Operational expenditure Attacker must similarly waste resources 12

13 PoW: Proof of Waste? Block proves (statistically) real-world waste Capital expenditure Operational expenditure Attacker must similarly waste resources That s not waste! 13

14 Environment-Friendly Alternatives Permissioned system (BFT) Centralized Internal waste Proof of Stake Different properties Useful resources (storage, prime numbers) Useful? For whom? Unfair 14

15 Partially Decentralized Guarantees Similar to PoW Assumptions Permissionless Trust secure-hardware manufacturer 15

16 Software Guard Extensions (SGX) 1. Confidentiality (incl. SRNG) 2. Integrity 3. Remote attestation Code & Data Untrusted Application Code output Untrusted Operating System & Hypervisor Trusted Processor Untrusted Hardware Attestation: Output Fingerprint signature B 16

17 Intel s Proof of Elapsed Time (PoET) Model: Partially decentralized Idea: Simulate PoW by sleeping. Pros: PoW-like guarantees Energy-waste-free 17

18 Intel s Proof of Elapsed Time Mining power not proportional to CPU value The Stale Chips Problem: Build a mining farm Old useless CPUs Waste hardware rather than power 18

19 Intel s Proof of Elapsed Time Individual CPUs can be compromised The Broken Chips Problem Intel proposes a simple statistical test. But 1. What is the adversary s advantage? 2. What is the cost of this test? 19

20 Proof of Useful Work

21 Proof of Useful Work Dedicate useful work rather than useless How to measure and prove? Memoryless random distribution Tunable difficulty O(1) validation All data on the blockchain 21

22 SGX Enclave Proof of Useful Work Run Useful Work Useful work, block header Instruction count n Simulate n Bernoulli tests Useful Work Result If success Proof of Useful Work 22

23 Hierarchical Attestation Only predefined programs? Alice s Enclave Bob s Enclave output Carols s Enclave 23

24 Hierarchical Attestation Only predefined programs? Hierarchical attestation! Compliance Checker output: Alice s Enclave Bob s Enclave output Carols s Enclave 24

25 Hierarchical Attestation Only predefined programs? Hierarchical attestation! Add instruction counting Return count 25

26 Useful Work client 2 Useful tasks 3 Useful results Miner TEE PoUW Enclave 2 4 Block template PoUW Blockchain agent 1 5 State New block Blockcha Netw 26

27 State Blockchain P2P Network 6 State Blockchain Agent Verifiers Content Compliance New block Effort 27

28 Performance 28

29 SGX Compromise Individual SGX instances might be broken Compromised SGX will attest to anything Compromise does not allow key forgery The Broken Chip problem 29

30 The Broken Chip Problem Statistically test likelihood of blocks from same CPU. Reject if unlikely. Too permissive: Attacker gains Too restrictive: Lose mining power 30

31 Attacker s Advantage Attacker s Advantage Block Count 31

32 Waste Wasted Proof of Work Block Count 32

33 Conclusion Blockchain security in partially-decentralized model Proof of Useful Work with TEE (SGX) PoC tool-chain Practical Performance Broken Chip Resilience Hierarchical Attestation Zhang, Eyal, Escriva, Juels, van Renesse. USENIX Security