While interviewing for a blockchain role, I was asked to clarify blockchain types for a non‑technical stakeholder.

Provide a concise, accurate comparison and relevant examples.

I described public blockchains as open networks where anyone can read/write (e.g., Bitcoin for decentralized payments). Private blockchains are restricted to a single organization, used for internal asset tracking. Permissioned blockchains combine openness with access control, such as Hyperledger Fabric for supply‑chain consortiums. I highlighted key properties: consensus, governance, and privacy.

The stakeholder understood the distinctions, enabling them to choose the appropriate platform for their project proposal.

During a technical screen I needed to demonstrate understanding of data integrity mechanisms.

Explain Merkle trees and their role in block validation.

I defined a Merkle tree as a binary hash tree where leaf nodes are transaction hashes and parent nodes are hashes of their children, culminating in a single root hash. I explained that the Merkle root is stored in the block header, enabling efficient verification of any transaction with a logarithmic proof, and supporting light clients and tamper‑evidence.

The interviewer confirmed my grasp of both the structure and its security benefits.

In a mid‑level interview I was asked about Ethereum's resource pricing model.

Explain gas mechanics and its security implications.

I described gas as a unit measuring computational work. Every operation has a gas cost; users specify a gas limit and gas price. The network deducts gas from the sender’s balance, halting execution when the limit is reached. This prevents infinite loops and denial‑of‑service attacks because malicious code cannot consume unlimited resources without paying. I also noted the role of the EIP‑1559 fee market in stabilizing prices.

The interviewer appreciated the depth of my answer and asked a follow‑up on gas optimization.

During a senior developer interview I needed to outline end‑to‑end contract delivery.

Describe each phase and best practices for upgrades.

I started with requirement gathering and threat modeling, then wrote Solidity code using Remix or VS Code with Hardhat. I emphasized unit testing with Mocha/Chai, integration tests on a local Ganache node, and static analysis via Slither. For deployment I used Hardhat scripts or Truffle migrations, signing transactions with a secure wallet (e.g., Ledger). I covered verification on Etherscan and post‑deployment monitoring. For upgrades I explained proxy patterns (Transparent or UUPS) and the importance of storage layout compatibility, using OpenZeppelin Upgrades plugin.

The interview panel noted my comprehensive approach and asked about handling storage collisions.

In a technical interview I was asked to demonstrate secure coding against reentrancy.

Explain mitigation techniques and show example code.

I listed the Checks‑Effects‑Interactions pattern, using a reentrancy guard modifier from OpenZeppelin, and limiting external calls. I provided a code snippet of a withdraw function that updates the balance before calling transfer, and an example of the nonReentrant modifier implementation. I also mentioned using pull‑payment patterns and avoiding low‑level call when possible.

The interviewer praised the practical example and asked about gas implications of the guard.

During a senior blockchain role interview I was tasked with outlining a robust token vesting solution.

Design a contract meeting cliff, linear vesting, and revocable features.

I described a contract storing beneficiary, total allocation, start time, cliff duration, vesting duration, and revocable flag. I used a function vestedAmount() that calculates vested tokens based on elapsed time, applying the cliff (no tokens before cliff). Linear vesting is computed as (elapsed‑cliff)/vestingPeriod * totalAllocation. A revoke() function callable by the owner transfers unvested tokens back to the owner if revocable. I referenced OpenZeppelin TokenVesting as a baseline and added events for Release, Revoked, and UpdatedBeneficiary.

The interviewers noted the completeness of the design and asked about edge‑case handling for premature releases.

In a blockchain security interview I needed to compare consensus models.

Explain technical differences and security implications.

I outlined that PoW relies on computational puzzles solved by miners, securing the network through hash power; security is proportional to energy cost and susceptibility to 51% attacks if hash power concentrates. PoS selects validators based on stake, reducing energy consumption; security hinges on economic penalties (slashing) and the cost of acquiring majority stake. I discussed finality, attack vectors (nothing‑at‑stake, long‑range attacks), and how PoS improves scalability but introduces new governance challenges.

The interviewer acknowledged the thorough comparison and asked about hybrid models.

During a pre‑deployment audit for a DeFi protocol I was asked to outline my audit methodology.

Provide a systematic audit process with tooling recommendations.

I described a multi‑phase approach: (1) Requirements review and threat modeling; (2) Static analysis using Slither, MythX, and Oyente to catch common patterns; (3) Manual code review focusing on access control, arithmetic, reentrancy, and upgradeability; (4) Unit and fuzz testing with Hardhat and Echidna; (5) Formal verification of critical functions using Certora or Solidity‑SMT; (6) Gas profiling with Tenderly; (7) Deployment on a testnet for integration testing; (8) Peer review and audit report generation. I highlighted documenting findings and remediation steps.

The interview panel noted the comprehensive checklist and asked about handling false positives from static tools.

In a senior DeFi interview I was asked to explain flash loan exploits and mitigation strategies.

Define flash loans, illustrate a typical attack, and propose defenses.

I explained that flash loans allow borrowing unlimited assets within a single transaction as long as the loan is repaid before the transaction ends. Attackers use them to manipulate price oracles, perform arbitrage, or drain liquidity pools. I described a classic attack on a vulnerable oracle where the attacker swaps a large amount, skews the price, borrows against the inflated price, and reverts. Defenses include using time‑weighted average price (TWAP) oracles, adding slippage checks, limiting loan size, employing reentrancy guards, and designing protocols to be atomic‑transaction resistant. I also mentioned using external price feeds with delay and circuit breakers.

The interviewer was impressed with the depth and asked about real‑world examples like the bZx attack.