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Consensus Mechanisms Flashcards

7 cards from real CBCP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. What is the primary security assumption behind Nakamoto consensus used in Bitcoin?

    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.

  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?

    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.

  3. What is 'finality' in the context of blockchain consensus, and which type is typically provided by BFT protocols?

    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.

  4. Which consensus mechanism is most suitable for a consortium blockchain where all participants are known and semi-trusted?

    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.

  5. What is the 'long-range attack' vulnerability specifically associated with Proof of Stake systems?

    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.

  6. What role does the 'leader election' step play in consensus protocols like HotStuff?

    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.

  7. In Ethereum's Gasper consensus (combining Casper FFG + LMD-GHOST), what does 'LMD-GHOST' determine?

    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.