Module L – Interactive Quiz – 20 Multiple-Choice Questions
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Q1Understand
What distinguishes “cost-of-attack” from “cost-of-corruption” in blockchain security?
Explanation
Zamfir (2015) defined cost-of-corruption as the provable capital destroyed when an attack is detected, distinct from the upfront execution cost.
Q2Apply
An attacker holds 40% of staked ETH. Using S = 3f × Bond, what fraction of the bond is burned on detection?
Explanation
3 × 0.40 = 1.20: the correlation penalty exceeds the bond, so 100% is burned (capped at the bond value).
Q3Analyze
Why is PoW attack capital “rentable” while PoS attack capital is “non-rentable”?
Explanation
NiceHash and similar platforms allow temporary hash power rental with no permanent commitment. No equivalent attack-capital lease market exists for PoS.
Q4Apply
A PoW attacker spends $10M on ASICs and fails to execute a 51% attack. Approximately what fraction of the $10M can be recovered?
Explanation
ASICs retain substantial market value and can be resold, unlike a burned PoS bond.
Q5Understand
In the expected payoff formula \mathbb{E}[payoff] = p · G - (1-p) · S - C_b, what does S represent?
Explanation
S is the slashed bond destroyed by the correlation penalty on detection; it is the dominant term at high stake fractions.
Q6Apply
Using S = 3f × Bond, what fraction of the bond is burned for an attacker holding exactly 10% of total stake?
Explanation
3 × 0.10 = 0.30, so 30% of the bond is burned. The correlation penalty is non-linear: it grows faster than the attacker’s stake.
Q7Apply
For which attacker stake fraction f does the correlation penalty S equal exactly the full bond?
Explanation
3 × 0.33 ≈ 1.00: at 33% the penalty equals 100% of the bond, meaning full capital loss.
Q8Analyze
Why does the expected payoff of a PoS attack grow more negative as the attacker accumulates more stake?
Explanation
S grows with 3f, while G (double-spend profit) is bounded by market liquidity. The tighter the inequality, the more irrational the attack.
Q9Analyze
What does “economic finality” mean in PoS, versus “probabilistic finality” in PoW?
Explanation
Casper FFG makes reversion provably costly (the attacker loses 33%+ of staked ETH). Nakamoto finality is probabilistic: deep reversal is expensive but never impossible in theory.
Q10Apply
If p = 0.95 and G = \10M, what must S exceed for the attack to be irrational (ignoring C_b$)?
Explanation
For \mathbb{E}[payoff] < 0: 0.95 × 10 < 0.05 × S, giving S > \190$M. This magnitude is achievable for large validators at 33%+ stake.
Q11Understand
What is “weak subjectivity” in Proof-of-Stake?
Explanation
Named by Buterin (2014): a new client needs one piece of external social information (the checkpoint) beyond what the protocol can prove from genesis.
Q12Understand
Why does PoW avoid the weak subjectivity problem?
Explanation
Energy expenditure makes PoW chain history unforgeable. In PoS, past block production was costless: old keys can simulate alternate histories without any energy cost.
Q13Analyze
An attacker uses keys from a validator set that withdrew 3 months ago. What attack can they mount against a newly syncing PoS node?
Explanation
Withdrawn validators retain their old signing keys. Checkpoints defeat this by providing an anchor more recent than the fork point.
Q14Understand
What is the approximate weak subjectivity period on Ethereum mainnet (as of 2023)?
Explanation
The Ethereum Foundation documentation specifies approximately 2 weeks; nodes offline longer than this must obtain a recent checkpoint before syncing.
Q15Apply
A user running an Ethereum node has been offline for 3 weeks. What must they do before trusting the chain?
Explanation
Three weeks exceeds the 2-week weak subjectivity period. The node must obtain a recent checkpoint to safely identify the honest chain.
Q16Analyze
Which dimension does this supplement add to L10’s PoW/PoS consensus comparison?
Explanation
L10 covers energy/TPS/finality speed. This deck adds: capital recoverability, finality type (probabilistic vs. economic), and attack-capital assembly time.
Q17Understand
What is the primary ongoing cost of PoS network security, as opposed to PoW?
Explanation
PoS validators earn newly issued ETH (inflation) as rewards. PoW miners earn block rewards plus fees but require continuous energy expenditure.
Q18Analyze
Why does a would-be PoS attacker face more entry friction than a PoW attacker?
Explanation
Assembling 33%+ of staked ETH is slow and publicly visible on-chain. NiceHash allows PoW attackers to assemble attack-scale hash power in hours with no on-chain trace.
Q19Apply
A PoS attacker holds 35% of staked ETH. The attack is detected and the correlation penalty applied. What is their capital position?
Explanation
3 × 0.35 = 1.05: the penalty exceeds the bond, so 100% of the bond is burned. Total capital loss.
Q20Evaluate
A designer claims: “Switch to PoW because PoS has a weak subjectivity problem.” What is the strongest economic counter-argument?
Explanation
Weak subjectivity is bounded and auditable. PoW’s recoverable attack capital (60-70% hardware resale) and rental market (NiceHash) reduce the economic cost of a failed attack relative to PoS’s burned bond.