Quiz: Consensus Mechanisms

Answer: (C) In an open, permissionless network, there is no identity verification. An attacker can spin up unlimited fake nodes to control the vote. Consensus mechanisms like PoW and PoS prevent Sybil attacks by tying voting power to a scarce resource (electricity or capital).

Answer: (B) The requirement is n > 3f, so 12 > 3f, meaning f < 4. The maximum integer value is f = 3. With 3 Byzantine nodes out of 12, the network has 9 honest nodes, satisfying 12 > 3 × 3 = 9.

Answer: (B) Pre-image resistance means you cannot reverse a hash to find the input. Miners must brute-force search for a nonce that produces a hash below the target. But once found, any node computes a single hash to verify it matches — exploiting the one-way asymmetry.

Answer: (A) Ethereum's PoS provides economic finality: once a block is finalized (after 2 epochs), reversing it would require at least 1/3 of validators to submit conflicting attestations, triggering slashing of their staked ETH. This makes reversal economically catastrophic, not physically impossible.

Answer: (C) Mining is a brute-force probabilistic search. With 19 leading zeros required, the expected number of attempts is 2¹⁹ ≈ 524,288, but in practice Bitcoin's difficulty is much higher (80+ leading binary zeros), requiring ~10²⁰ attempts globally. The miner must keep trying.

Answer: (D) The adjustment formula is: new_difficulty = old_difficulty × (target_time / actual_time) = old × (14 days / 11.2 days) = old × 1.25. Difficulty increases by 25% to compensate for blocks arriving faster than the 10-minute target.

Answer: (D) Double-voting (signing two blocks at the same height) is a slashable offense. The protocol automatically detects conflicting signatures, burns a portion of the validator's stake, and initiates forced exit. This economic punishment is what gives PoS its security guarantee.

Answer: (B) Under the longest chain rule, a temporary fork exists until one chain grows longer. The next miner to find block 700,002 will build on whichever version of 700,001 they received first. The extended chain becomes canonical; the shorter chain is orphaned.

Answer: (C) PBFT is designed for permissioned networks with a known set of participants. It provides deterministic finality (no waiting for confirmations), tolerates up to 3 Byzantine nodes out of 10, and does not waste energy on mining. A shared database sacrifices auditability and fault tolerance.

Answer: (A) Miners cannot change the difficulty target (it is set by the protocol). The halving permanently reduces the reward; it never reverts. Ethereum no longer has mining (post-Merge PoS). The only option is reducing costs: more efficient hardware (lower J/TH) and cheaper electricity.

Answer: (D) A consensus mechanism's Sybil resistance depends on tying voting power to a genuinely scarce resource. PoW uses electricity (expensive); PoS uses capital (expensive). Social media followers can be bought cheaply by the thousands, allowing an attacker to accumulate disproportionate influence at low cost.

Answer: (C) Bitcoin requires ~60 minutes for 6 confirmations. Ethereum PoS requires ~12.8 minutes for finality. Only Tendermint (and similar BFT protocols like Avalanche) achieve sub-10-second deterministic finality, making them suitable for real-time payment applications.

Answer: (C) High hardware requirements create a barrier to entry: fewer individuals and organizations can afford to run validator nodes. This concentrates validation power among well-funded entities, reducing decentralization. Solana achieves scalability by accepting this centralization trade-off.

Answer: (A) Both PoW and PoS exhibit economies of scale. In PoW, wealthier entities buy more ASICs and secure bulk electricity deals. In PoS, wealthier validators stake more and earn proportionally more rewards. The "rich get richer" concern applies to both mechanisms, though PoS at least avoids the environmental externality.

Answer: (B) With 40% hash power vs. 60% honest, the attacker's expected chain growth is 40/60 ≈ 0.67 blocks per honest block. After k confirmations, the probability of catching up decreases exponentially: P ≈ (40/60)^k. After 6 confirmations, P ≈ 0.67⁶ ≈ 9%. This is why merchants wait for confirmations.

Answer: (B) If more than 1/3 of validators go offline, finality halts (the 2/3 threshold cannot be met). Inactivity leaks gradually reduce offline validators' stakes until the online validators represent 2/3 of the remaining total stake, restoring the network's ability to finalize blocks.

Answer: (D) Censorship resistance comes from decentralization and anonymity. Network A's 10,000 anonymous miners span multiple jurisdictions; blocking a transaction requires convincing all of them. Network B's 20 identified banks can be ordered by a regulator or court to exclude specific transactions. This is the fundamental decentralization vs. permissioned trade-off.

Answer: (A) Layer 2 solutions (Lightning Network, rollups) move transaction execution off-chain. Thousands of transactions occur on L2, and only a summary (or cryptographic proof) is posted to L1. This preserves L1's decentralization and security while dramatically increasing effective throughput.

Answer: (D) When China banned crypto mining in 2021, roughly 50% of global hash power went offline. The network experienced slower blocks temporarily, but difficulty adjusted downward within two weeks. Miners relocated to the US, Kazakhstan, and other countries. Bitcoin's protocol is jurisdiction-agnostic — it adapts automatically.

Answer: (A) PoW is eliminated by the 3-second finality requirement (Bitcoin: ~60 min). Ethereum PoS is eliminated by both the 3-second requirement (~13 min finality) and 1,000 TPS (~30 TPS on L1). PBFT provides fast finality but is permissioned, violating the openness requirement. Avalanche achieves sub-second finality, thousands of TPS, and permissionless participation through its novel repeated sub-sampled voting mechanism.