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What is Finality?

When you send a transaction, how long do you have to wait before you’re confident it can’t be reversed? This waiting time is called finality. Two Types of Finality:
  1. Statistical Finality: Mathematical certainty your transaction is permanent (barring a 51% attack)
  2. Economic Finality: The cost to reverse your transaction exceeds any attacker’s benefit
Why Faster Finality Matters:
  • Better user experience: No waiting 10+ minutes for Bitcoin confirmations
  • Enables commerce: Merchants can accept payments instantly
  • Reduces uncertainty: Clear when transactions are truly final

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The Challenge: Quai’s Multi-Chain Architecture

Quai Network uses a hierarchy of blockchains:
  • Prime chains: Main coordination chains (slower, more secure)
  • Region chains: Regional coordination chains
  • Zone chains: Individual transaction chains (faster, where users interact)
The Problem: In traditional systems, transactions on fast chains (zones) aren’t final until confirmed by slow chains (prime). This creates uncertainty.

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The Withholding Attack Problem

What’s a Withholding Attack? Imagine a miner finds a valid block but doesn’t immediately broadcast it. Instead, they hold it back while other miners waste energy mining the previous block. In Single Chains (like Bitcoin):
  • Attacker holds back a block for ~10 minutes
  • Minimal impact since no transactions process between blocks
  • Eventually another miner finds a block, making the withheld block worthless
In Multi-Chain Systems (Traditional Approach):
  • Zone chains process transactions continuously
  • But those transactions aren’t final until prime chain confirms
  • Attacker could hold back a prime block, keeping zone transactions uncertain
  • Much more disruptive than single-chain attacks

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PoEM’s Solution: Bottom-Up Finality

Traditional Hierarchy (Top-Down):
  • Prime chains lead, zone chains follow
  • Zone transactions wait for prime confirmation
  • Vulnerable to prime chain withholding attacks
PoEM Hierarchy (Bottom-Up):
  • Zone chains can achieve finality independently
  • Prime chains follow zone chain entropy accumulation
  • Withholding attacks become ineffective
How This Works:
  • Zone chains remove entropy faster than prime chains (due to higher frequency)
  • Even the “luckiest” possible prime block can’t outweigh zone chain accumulation for long
  • Transactions achieve finality in seconds, not minutes

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Finality Comparison

SystemStatistical Finality TimeMethod
Bitcoin10+ minutes (1 block)Wait for longest chain
Ethereum12+ minutes (2 epochs)Wait for 2/3 validator approval
Quai PoEM~5 seconds (1 zone block)Entropy accumulation measurement
Why PoEM is Faster:
  • Objective measurement: Entropy is mathematically scarce, not subjective
  • Independent chains: Zone finality doesn’t depend on prime blocks
  • Precise calculation: Measures exact work, not arbitrary thresholds
Learn more about the mathematical details of this calculation.

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Economic Finality: Real-World Security

What is Economic Finality? The point where reversing your transaction would cost an attacker more than they could possibly gain. Real-World Examples:
  • Coffee purchase ($5): Economically final almost instantly
  • Car purchase ($50,000): May need 30+ minutes for full economic security
  • House purchase ($500,000): Could require hours of confirmations
Factors Affecting Economic Finality:
  • Transaction value: Higher value = longer wait time needed
  • Network hashrate: More miners = better security = faster finality
  • Pending transactions: Network congestion affects attack costs
  • Market conditions: Token price volatility impacts attack economics

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Built-in Economic Finality Tool

Quai Network includes a smart finality calculator that: Analyzes Network Conditions:
  • Current hashrate and mining distribution
  • Pending transaction volumes and fees
  • Historical attack costs and patterns
Provides Custom Recommendations:
  • Instant decisions for small transactions
  • Precise wait times for large transfers
  • Real-time updates as conditions change
Use Cases:
  • Merchants: Know exactly when payments are safe
  • Exchanges: Minimize deposit wait times while maintaining security
  • Users: Understand the security level of their transactions
Economic finality is dynamic - the same transaction might need different wait times depending on current network conditions and market factors.