CBCP Consensus Mechanisms 3 — Questions and Answers
Question 1: What is the primary security assumption behind Nakamoto consensus used in Bitcoin?
- At least 67% of validators must be honest
- An attacker controlling less than 50% of hash rate cannot consistently outpace the honest chain (Correct answer)
- All nodes must be known and identified
- Transactions are confirmed only after 1 block
Correct answer: An attacker controlling less than 50% of hash rate cannot consistently outpace the honest chain
Nakamoto consensus assumes that as long as honest miners control the majority of hash rate, the longest chain produced by honest nodes will outpace any attacker's chain.
Question 2: In a Practical Byzantine Fault Tolerant (PBFT) system with 3f+1 nodes, what is the maximum number of faulty nodes (f) it can tolerate?
- One-third of total nodes minus one (Correct answer)
- One-half of total nodes
- One-quarter of total nodes
- Two-thirds of total nodes minus one
Correct answer: One-third of total nodes minus one
PBFT requires 3f+1 total nodes to tolerate f Byzantine faults, meaning it can tolerate up to ⌊(n-1)/3⌋ faulty nodes.
Question 3: What is 'finality' in the context of blockchain consensus, and which type is typically provided by BFT protocols?
- Finality means transactions are irreversible; BFT provides probabilistic finality
- Finality means transactions are irreversible; BFT provides absolute (deterministic) finality (Correct answer)
- Finality means blocks are mined faster; BFT provides economic finality
- Finality refers to fee settlement; BFT provides instant fee clearing
Correct answer: Finality means transactions are irreversible; BFT provides absolute (deterministic) finality
BFT-based protocols provide deterministic (absolute) finality — once a block is committed, it cannot be reverted, unlike the probabilistic finality of PoW chains.
Question 4: Which consensus mechanism is most suitable for a consortium blockchain where all participants are known and semi-trusted?
- Proof of Work
- Proof of Burn
- Practical Byzantine Fault Tolerance (PBFT) (Correct answer)
- Nakamoto Longest Chain
Correct answer: Practical Byzantine Fault Tolerance (PBFT)
PBFT is well-suited for consortium blockchains with a known, limited set of semi-trusted validators due to its deterministic finality and low energy consumption.
Question 5: What is the 'long-range attack' vulnerability specifically associated with Proof of Stake systems?
- Attackers stake tokens on competing forks to earn double rewards
- An attacker with old private keys can rewrite history from a past point where they held significant stake (Correct answer)
- Validators collude to block specific transactions indefinitely
- Staking pools accumulate enough stake to control 51% of voting power
Correct answer: An attacker with old private keys can rewrite history from a past point where they held significant stake
In a long-range attack, an adversary uses old keys (from when they had large stake) to rewrite blockchain history from a past checkpoint, which PoW naturally prevents through cumulative work.
Question 6: What role does the 'leader election' step play in consensus protocols like HotStuff?
- It selects which user's transaction gets processed first
- It determines which validator proposes the next block for others to vote on (Correct answer)
- It assigns mining rewards to the winning node
- It identifies which node will audit the current block
Correct answer: It determines which validator proposes the next block for others to vote on
Leader election designates a single proposer node to suggest the next block, reducing communication complexity by funneling proposals through one coordinator.
Question 7: In Ethereum's Gasper consensus (combining Casper FFG + LMD-GHOST), what does 'LMD-GHOST' determine?
- The penalty amount for slashable offenses
- The canonical chain head by following the fork with the most recent validator votes (Correct answer)
- The minimum stake required to become a validator
- The block reward distribution formula
Correct answer: The canonical chain head by following the fork with the most recent validator votes
LMD-GHOST (Latest Message Driven Greediest Heaviest Observed SubTree) is the fork-choice rule that selects the chain head based on the latest votes from validators.
CBCP Consensus Mechanisms 3
What is the primary security assumption behind Nakamoto consensus used in Bitcoin?