Blockchain Protocol Engineer Interview Questions

Option B is strongest because it precisely describes the EVM's execution model (256-bit stack machine, stack depth, memory layout, storage trie), names specific opcode costs with EIP references (EIP-2929 for cold/warm access), explains gas exhaustion semantics, and covers the critical distinction between CALL, DELEGATECALL, and STATICCALL with the proxy pattern use case. Option A is definitionally correct but far too thin for a protocol engineer role. Option C describes compilation and execution at a high level without any opcode-level or cost-model detail. Option D addresses the economic incentive layer but does not answer the question about execution mechanics. Blockchain Protocol Engineer interview best practice: always separate the three EVM storage zones (stack, memory, storage) and explain their cost implications, as storage access patterns are the primary lever for gas optimisation in smart contracts.

Option B is strongest because it names precise protocol mechanisms (Nakamoto consensus, Casper FFG, LMD-GHOST, Gasper), quantifies finality timelines, explains the attack threshold differences (51% vs 33%/66%), covers slashing as an economic deterrent, and addresses the long-range attack problem and its mitigation via weak subjectivity. Option A reduces the comparison to energy efficiency, which is a policy argument, not a protocol design analysis. Option C is factually accurate but provides no technical depth on finality models or security assumptions. Option D is diplomatically balanced but offers no substantive protocol analysis. Blockchain Protocol Engineer interview best practice: always distinguish probabilistic finality (PoW longest-chain) from economic finality (PoS slashing) and quantify the attack cost in terms of percentage of stake or hash rate controlled, as this is the metric used when assessing network security budgets.

Option B is strongest because it walks through the full compilation pipeline (arithmetic circuit → R1CS → QAP → polynomial commitment scheme), explains the trusted setup mechanics and the "one honest participant" safety model, contrasts Groth16 and PLONK on setup universality, then pivots to compare SNARKs and STARKs on proof size, verification cost, and post-quantum security — using specific size and latency numbers. Option A is a correct but extremely thin definition. Option C describes the trusted setup ceremony but does not explain what zk-SNARKs are, how they work, or how to choose between SNARK and STARK. Option D makes a blanket claim ("more secure") without acknowledging the proof-size/verification-cost trade-off that makes SNARKs preferable for on-chain verification. Blockchain Protocol Engineer interview best practice: when explaining ZK systems, always trace the pipeline from arithmetic circuit to on-chain verifier and quantify proof size and EVM verification gas cost, as these are the dominant engineering constraints in L2 rollup design.

Option B is strongest because it derives the AMM swap formula algebraically, gives the exact fee tier, explains IL with the precise mathematical formula (2√r/(1+r) - 1) and example values at 2x and 5x price moves, and then articulates how Uniswap v3's virtual reserves model changes the capital efficiency and IL profile — including the NFT LP token change and the active market-maker analogy. Option A is accurate but provides no mathematics, no IL formula, and no v3 detail. Option C correctly defines IL directionally but without any formula or magnitude quantification, which is insufficient for a protocol engineer role. Option D describes v3 benefits but omits the higher IL severity in concentrated ranges and the active management burden, presenting an incomplete picture. Blockchain Protocol Engineer interview best practice: always state the IL formula and a concrete example price movement when discussing AMMs, and explicitly address the capital efficiency vs active management trade-off in v3 to demonstrate you understand why passive LPs often underperform in concentrated ranges.

Option B is strongest because it explains the data availability layer shared by both approaches, quantifies the optimistic challenge window (7 days), names the production fraud proof systems (Arbitrum BOLD, Optimism Cannon), gives the ZK proof verification cost (~500k gas), distinguishes zkEVM type equivalence levels (Type 1/2/3), and explains proving latency and its mitigations (recursive proofs, ASIC provers). It also gives a concrete decision heuristic. Option A is accurate but entirely qualitative — it names the concepts without any timelines, costs, or production system names. Option C correctly notes the finality difference but provides no depth on challenge window mechanics, trust assumptions, or proving cost. Option D makes a prediction about ZK replacing Optimistic without any technical analysis to support it and ignores the engineering trade-offs that make each suitable for different use cases today. Blockchain Protocol Engineer interview best practice: always quantify the challenge window duration and the ZK proof verification gas cost, and name at least two production implementations of each type to demonstrate you have hands-on awareness of the ecosystem.