Bitcoin Consensus Mechanism——SYSU SSE Blockchain 5th lecture(English Version)

news2024/11/27 17:55:35

Consistency: All changes are seen by all nodes at the same time.
Availability: Systems return a response within an acceptable time frame.
Partition Tolerance: The system continues to function when network segments fail to communicate. Systems can be partitioned into sub-networks that continue to function independently.

Availability ≠ Correctness: You can only achieve two out of the three CAP properties in a distributed system.
CA/P: Focuses on consistency and availability but lacks partition tolerance.
- Implementation: All data is stored on a single node, and other nodes read from this main node.
- Note: Not considered a distributed system.
CP/A: Focuses on consistency and partition tolerance at the cost of availability.
- Implementation: Each node stores data, with the primary focus on ensuring consistency.
AP/C: Focuses on availability and partition tolerance at the cost of consistency.
- Implementation: Each node stores data, with the primary focus on ensuring availability.

Note: Since distributed systems must have Partition Tolerance (P), they must choose between Consistency (C) and Availability (A).

Part 2: Why Bitcoin needs Consensus

BFT-based (Byzantine Fault Tolerance)
- Feature: Low computational requirements
- Participants: Permissioned, limited trust
- Applicability: Alliance/Private Chains
Leader Election-based
- Example: Proof of Work (PoW)
- Feature: Computationally intense
- Participants: Permissionless, anonymous, no trust
- Applicability: Public Chains

Problem: Uncertain transmission of information due to possible traitors
Goal: To maintain consistency in a distributed system
Solution:
- Simplified to a Generals and Lieutenants model
- Loyal Lieutenants follow the same command, traitors may disrupt
Fault Tolerance: Malicious nodes must be less than 1/3 of the total nodes

Consequence: Makes Byzantine Fault Tolerance unsuitable for Bitcoin
Method: Creating multiple identities to cast multiple votes
Why Feasible:
- Bitcoin is a decentralized, anonymous service
- Low cost to create multiple identities
Example: Double-spending attack
Defense: Resource-based voting through Proof of Work (PoW)

Mining Process: Combines previous block's hash, Merkle root of new transactions, a nonce, and a timestamp.
Success Criterion: SHA(Merkle Root + Previous Block Hash + Timestamp + Nonce) < target
- Number of leading zeros indicates difficulty level

51% Attack: Control over more than half of the computational power can result in a fake chain being accepted.
Prevention: Wait for 6 block confirmations before proceeding with transactions.

Challenging: Difficult to solve but easy to verify
Fairness: No shortcuts
Randomness: Ensures a decentralized and unpredictable process
Stability: Enables the stable operation of the blockchain with multiple-party participation