Blockchain Interview Questions &amp; Answers

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A blockchain is a distributed, append-only ledger where records are grouped into blocks and each block stores the hash of the previous block, forming a chain. Many independent nodes keep a copy and agree on new blocks through a consensus protocol, so no single party controls the data. Changing an old block would change its hash and break every later link, which makes tampering easy to detect. Bitcoin and Ethereum are the best known public examples.

How is a blockchain different from a traditional database?

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A traditional database is controlled by one administrator who can create, update and delete rows, and trust comes from that owner. A blockchain is shared by many parties, is append-only, and trust comes from cryptography and consensus. The trade-off is performance: a database handles thousands of writes per second with low latency, while a public chain is slower and costlier. Use a blockchain only when several parties who do not fully trust each other must share one record.

What does a block contain and what is inside a block header?

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A block has a header and a body. The body holds the list of transactions. The header holds metadata: the previous block hash, a timestamp, the Merkle root (or state and transaction roots in Ethereum) summarising the transactions, and consensus fields such as the difficulty target and nonce in Bitcoin. Because the header commits to all transactions through the root hash, nodes can verify a block's integrity by hashing just the header.

Explain public, private and consortium blockchains.

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A public blockchain such as Bitcoin or Ethereum lets anyone read, transact and run a validator. A private blockchain is run by one organisation that controls who joins, useful for internal audit trails. A consortium (permissioned) blockchain is governed by a group of organisations, for example several banks sharing trade-finance data on Hyperledger Fabric. Public chains give maximum decentralisation; permissioned chains give privacy, known identities and higher throughput.

How does a blockchain achieve immutability?

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Immutability comes from hash linking plus consensus. Each block includes the previous block's hash, so editing a past transaction changes that block's hash and every block after it. An attacker would have to redo the work or stake for all those blocks faster than the honest network, which is economically impractical on a large chain. Strictly it is tamper-evident and practically immutable rather than impossible to change; a hard fork by social agreement can still rewrite history.

You open a transaction on a block explorer. Which fields do you check and what do they mean?

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Check the status (success or failed), the block number and confirmations, from and to addresses, and the value transferred. For contract calls, check the decoded input data and emitted event logs, which show token transfers. Check the gas limit, gas used and effective gas price, which together give the fee paid, plus the nonce. If the status failed, the revert reason tells you why, while fees are still charged for gas used.

How would you set up a local blockchain for development and testing?

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For EVM work, run a local node with Anvil from Foundry or the Hardhat network. Each gives about ten pre-funded test accounts, instant mining and console logging. You can fork mainnet at a given block to test against real contracts and balances without spending money. For integration tests, deploy to a public testnet such as Sepolia, using faucet ETH. Keep deploy scripts and RPC URLs in config so the same code targets local, testnet and mainnet.

Why do you want to build a career in blockchain?

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Use a specific, sincere answer. For example: I first got interested when I built a small token contract in college and saw that code could hold and move value without a middleman. Since then I have completed Solidity projects, learnt Foundry testing and read audit reports. I like that the field mixes cryptography, distributed systems and finance. I want to grow as a smart contract developer and help build secure, real-world applications such as payments, tokenisation or supply chain.

What is the difference between a full node, a light node and an archive node?

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A full node downloads and validates every block and keeps the current state, so it can verify everything independently. A light node stores only block headers and asks full nodes for Merkle proofs of the data it needs, which suits mobile wallets. An archive node keeps all historical states, so you can query a balance at any past block; indexers and block explorers need it, but it requires many terabytes of storage.

What is double spending and how does a blockchain prevent it?

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Double spending is trying to spend the same digital money twice, for example sending the same coins to two merchants. A blockchain prevents it by keeping one shared ordering of transactions agreed through consensus. Once a transaction is included in a block, any conflicting transaction that spends the same funds is rejected as invalid. Merchants wait for several confirmations so that the chance of the block being replaced in a reorg becomes negligible.

Compare the UTXO model and the account model.

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In the UTXO model used by Bitcoin, coins exist as unspent transaction outputs; a transaction consumes some outputs and creates new ones, which makes parallel validation and privacy easier. In the account model used by Ethereum, the state stores a balance and nonce per account, which is simpler for smart contracts and wallets. The trade-off: UTXO avoids replay issues and scales verification well, while accounts make complex stateful logic far easier to write.

What is the difference between a soft fork and a hard fork?

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A soft fork tightens the rules so that new blocks are still valid for old nodes; it is backward compatible, like Bitcoin's SegWit upgrade. A hard fork changes the rules so that old nodes reject new blocks, so every node must upgrade or the chain splits, as with Ethereum and Ethereum Classic in 2016. Planned network upgrades on Ethereum, such as Dencun and Pectra, are coordinated hard forks that all clients adopt together.

What is the blockchain trilemma?

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The trilemma says a blockchain finds it hard to maximise decentralisation, security and scalability at the same time on a single layer. Raising block size or using few validators increases throughput but reduces decentralisation. Ethereum's answer is a rollup-centric roadmap: keep the base layer decentralised and secure, and move execution to Layer 2 rollups that post data back to Layer 1. Many high-throughput chains instead accept fewer, more powerful validators.

What is a mempool?

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The mempool is each node's waiting area for valid transactions that have been broadcast but not yet included in a block. Block producers usually pick transactions paying the highest fees or priority tips. Every node has its own mempool, so contents can differ slightly. Because pending transactions are public, bots can watch the mempool and front-run trades, which is why private transaction relays and order-flow auctions are used for sensitive transactions.

What are the main components of Hyperledger Fabric?

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Hyperledger Fabric is a permissioned enterprise framework. Peers hold the ledger and run chaincode (smart contracts written in Go, Java or JavaScript). Orderers, usually using Raft, sequence transactions into blocks. A Membership Service Provider issues identities through certificate authorities. Channels give separate private ledgers to subsets of members, and private data collections hide data inside a channel. Fabric uses an execute-order-validate flow with endorsement policies instead of mining.

An HR team wants to store employee records on a blockchain. Would you recommend it?

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Usually no. Employee records are owned by one organisation, change often and contain personal data that must be correctable and deletable under the DPDP Act 2023. A normal database with role-based access and audit logs is cheaper, faster and compliant. I would ask who else needs to trust the data. If several employers or background-check agencies must verify certificates, I would store only hashes of credentials on a permissioned chain and keep the actual data off-chain.

Design a blockchain solution for farm-to-store supply chain traceability. What goes on-chain?

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I would use a permissioned or low-cost chain where farmers, transporters, warehouses and retailers are known members. On-chain I would record batch IDs, ownership handovers, timestamps, signer identity and hashes of certificates or sensor data. Large documents, images and IoT streams stay off-chain in object storage or IPFS, referenced by hash. QR codes on packs link consumers to the trail. The main risk is bad input data, so I would add signed device readings and audits.

A user's transaction has been pending for an hour. How do you troubleshoot it?

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First check it on a block explorer: if it is not found, it was never broadcast. If pending, the fee is probably below current market rates, or an earlier transaction with a lower nonce is stuck and blocking it. On Ethereum you fix it by sending a replacement with the same nonce and a higher max fee and priority fee, or cancel by sending zero ETH to yourself with that nonce. Then make the wallet estimate fees dynamically.

Write a minimal Python example showing how blocks are linked by hashes.

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Each block stores the previous hash, so changing any earlier data breaks the chain. import hashlib, json def block_hash(block): data = json.dumps(block, sort_keys=True).encode() return hashlib.sha256(data).hexdigest() chain = [{'index': 0, 'data': 'genesis', 'prev': '0'}] for i, tx in enumerate(['A pays B 5', 'B pays C 2'], 1): chain.append({'index': i, 'data': tx, 'prev': block_hash(chain[-1])}) valid = all(chain[i]['prev'] == block_hash(chain[i-1]) for i in range(1, len(chain))) print(valid)

How do you get the latest block number from an Ethereum node using JSON-RPC?

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Send a JSON-RPC POST request to the node or provider URL. The result is a hex string that you convert to decimal. curl -s -X POST https://rpc.example.org \ -H 'Content-Type: application/json' \ -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}' # Response: {"jsonrpc":"2.0","id":1,"result":"0x15d3a1c"} # In Python: int('0x15d3a1c', 16) Other common methods are eth_getBalance, eth_call and eth_getTransactionReceipt.

Tell me about a time you explained blockchain to a non-technical stakeholder.

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Situation: a business head wanted 'blockchain' in a vendor portal because competitors mentioned it. Task: help him decide. Action: I compared it to a shared online spreadsheet that nobody can secretly edit, then showed cost, speed and privacy trade-offs against a normal database with audit logs. Result: we agreed to keep the portal on a database and anchor monthly hashes of invoices on a chain for proof, saving months of work while meeting the trust goal.

What is finality, and how does probabilistic finality differ from deterministic finality?

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Finality is the guarantee that a confirmed transaction will not be reversed. In proof of work, finality is probabilistic: each new block on top makes reversal exponentially less likely, so exchanges wait for several confirmations. In BFT-style and Ethereum proof of stake, finality is economic and deterministic: once two-thirds of stake attests to a checkpoint, reverting it would require burning at least one-third of all staked ETH. Ethereum finalises in about two epochs, roughly 13 minutes.

A product team wants to store user documents and personal details directly on a public chain. What do you advise?

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Do not put personal data on a public chain. Data there is permanent, public and replicated globally, which conflicts with the right to erasure under India's DPDP Act and GDPR, and storage is extremely expensive. Store documents encrypted in off-chain storage, keep only a salted hash or commitment on-chain for integrity, and control access through the application. If the data must be deleted, delete the off-chain copy and the encryption key; the hash alone reveals nothing.

A group of banks wants a shared KYC ledger. Would you choose a public chain, a Layer 2 or Hyperledger Fabric?

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For shared KYC among regulated banks I would lean to a permissioned framework such as Hyperledger Fabric or a private EVM network. Members are known, data is confidential, and regulators expect access control and data residency in India. Fabric channels and private data collections restrict who sees what, and Raft ordering gives fast finality without gas fees. A public chain or L2 fits only if we need open verification, and even then only hashes and attestations would go on it.

Your exchange credited a deposit, but the block was later dropped in a reorg and the funds vanished. How do you prevent this?

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Credit deposits only after enough confirmations for the chain's risk profile, and on Ethereum prefer waiting for finalized or at least safe block tags rather than the latest block. Make the deposit indexer reorg-aware: track block hashes, detect when a parent hash changes, roll back credits from orphaned blocks and reprocess. Scale required confirmations with deposit size, and alert on unusually deep reorgs on smaller chains, which can signal a 51 percent attack.

What is a cryptographic hash function and what properties must it have?

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A hash function maps any input to a fixed-size output, such as 256 bits. For blockchain it must be deterministic, fast to compute, pre-image resistant (you cannot find the input from the hash), second pre-image resistant, and collision resistant (finding two inputs with the same hash is infeasible). It should also show the avalanche effect, where a one-bit change gives a completely different hash. Blocks, addresses, Merkle trees and proof of work all depend on these properties.

How is public-key cryptography used in blockchain?

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Each user has a private key, kept secret, and a public key derived from it. The private key signs transactions, and anyone can verify the signature with the public key, proving the owner authorised the transfer without revealing the private key. Addresses are derived from public keys. Because control of funds equals control of the private key, losing it means losing access, and leaking it means anyone can spend the funds.

What is a digital signature and what does it guarantee?

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A digital signature is a value produced by signing a message hash with a private key. Anyone with the public key can verify it. It guarantees authentication (the holder of the key signed it), integrity (the message was not changed, since any change breaks verification) and non-repudiation (the signer cannot later deny it). In blockchain, every transaction is signed, so nodes accept a transfer only if the signature matches the sending account.

What is a nonce in blockchain? Explain both common meanings.

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In proof of work, the nonce is a number miners change repeatedly in the block header until the header hash falls below the difficulty target. In Ethereum's account model, the account nonce is a counter of transactions sent from an address; each transaction must use the next nonce, which orders transactions and prevents replay. A stuck transaction with a low nonce blocks all later ones from that account until it is mined or replaced.

What is a hash collision and why does it matter for blockchains?

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A collision is when two different inputs produce the same hash. If attackers could find collisions, they could swap one transaction or block for another with the same hash and break integrity, Merkle proofs and address uniqueness. For SHA-256 and Keccak-256, finding a collision needs about 2^128 work, which is infeasible. This is why older broken hashes such as MD5 and SHA-1 must never be used in blockchain systems.

A user lost their private key and seed phrase. Can you recover their funds?

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No. In a self-custody wallet, nobody, including the developer, can recover a lost private key; deriving it from the address is computationally impossible. If they still have the seed phrase, the wallet can be restored on any compatible app. To reduce future risk, suggest a hardware wallet, a written seed phrase stored in two safe locations, or a smart contract wallet with social recovery or passkeys, where trusted guardians can rotate the signing key.

How do you compute a keccak256 hash in Solidity and in ethers.js?

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Solidity: bytes32 h1 = keccak256(abi.encodePacked('hello')); bytes32 h2 = keccak256(abi.encode(addr, amount)); ethers v6: import { keccak256, toUtf8Bytes, solidityPackedKeccak256 } from 'ethers'; const h = keccak256(toUtf8Bytes('hello')); const p = solidityPackedKeccak256(['address','uint256'], [addr, amount]); Prefer abi.encode over abi.encodePacked when hashing several dynamic values, because packed encoding can cause collisions.

How do you keep up with security and cryptography developments in Web3?

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I follow a routine. Weekly I read new audit reports and post-mortems of exploits, and I try to reproduce one in Foundry. I track Ethereum Improvement Proposals and upgrade notes, read Solidity release notes, and practise on capture-the-flag platforms such as Ethernaut and Damn Vulnerable DeFi. I also keep a personal notes file of attack patterns. This keeps my knowledge practical rather than just theoretical, and I share summaries with my team.

What is the difference between SHA-256 and Keccak-256?

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SHA-256 is from the SHA-2 family and uses a Merkle-Damgard construction; Bitcoin uses double SHA-256 for block hashes and transaction IDs. Keccak-256 is based on the sponge construction and is what Ethereum uses for addresses, function selectors, event topics and storage slots. Ethereum's Keccak-256 is the original Keccak submission and differs from the final NIST SHA3-256 in padding, so the two give different outputs for the same input.

What is ECDSA and why does Ethereum use the secp256k1 curve?

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ECDSA is the Elliptic Curve Digital Signature Algorithm. It gives strong security with short keys: a 256-bit key is roughly as strong as a 3072-bit RSA key, which keeps transactions small. Bitcoin and Ethereum use the secp256k1 curve, whose simple structure allows efficient computation. A signature has values r and s plus a recovery id v, which lets nodes recover the signer's public key from the signature, so the sender address need not be included separately.

How is an Ethereum address derived from a private key?

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Start with a random 256-bit private key. Multiply it by the secp256k1 generator point to get a 64-byte uncompressed public key. Take the Keccak-256 hash of that public key and keep the last 20 bytes; that is the address, shown as 0x plus 40 hex characters. EIP-55 adds a mixed-case checksum so typos can be detected. The process is one-way: you cannot get the private key back from the address or public key.

What is a Merkle tree and what is a Merkle proof?

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A Merkle tree hashes data items in pairs, level by level, until one root hash remains. The root summarises all items: changing any item changes the root. A Merkle proof is the list of sibling hashes on the path from one item to the root. With it, a light client can prove that a transaction is in a block using about log2(n) hashes instead of downloading all transactions. Airdrop allowlists use the same trick on-chain.

What are seed phrases and HD wallets (BIP-39, BIP-32, BIP-44)?

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BIP-39 encodes random entropy as a 12 or 24 word mnemonic, which is turned into a binary seed. BIP-32 defines hierarchical deterministic wallets that derive a tree of key pairs from that seed. BIP-44 standardises the derivation path, for example m/44'/60'/0'/0/0 for the first Ethereum account. So one backed-up seed phrase can restore unlimited addresses across chains. Whoever knows the phrase controls every derived account, so it must never be stored online.

Compare multisig wallets and MPC wallets.

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A multisig wallet is a smart contract, such as a Safe, that needs M of N owner signatures to execute a transaction; rules are visible on-chain and easy to audit, but every action costs extra gas and works per chain. An MPC wallet splits one private key into shares held by different parties who jointly produce a normal signature; it looks like a regular address, works on any chain and allows key rotation, but the security depends on the off-chain implementation.

Users complain they must pay gas to approve tokens before every trade. How can signatures help?

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Use off-chain signed approvals. With EIP-2612 permit, the user signs a typed EIP-712 message granting an allowance, and the dApp submits it with the trade in one transaction, so no separate approve call is needed. For tokens without permit, Permit2-style contracts give the same experience after one approval. EIP-712 shows readable fields in the wallet, reducing phishing risk. Always include a nonce, deadline, chain ID and verifying contract to prevent replays.

How would you check on-chain whether a user is in a 10,000-address allowlist without storing all addresses?

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Build a Merkle tree off-chain from the allowlisted addresses (hashing each leaf, ideally double-hashed with amounts if needed) and store only the 32-byte root in the contract. Each user submits a proof of about 14 hashes for 10,000 leaves. The contract recomputes the root with OpenZeppelin MerkleProof.verify and compares it. This costs one storage slot instead of 10,000, and you can update the list by changing the root.

A developer accidentally pushed a private key to a public GitHub repo. What do you do?

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Treat the key as compromised immediately, because bots scan GitHub within seconds. Move all funds and tokens to a new secure wallet, revoke token approvals, and if the key owns contracts, transfer ownership or roles to a new multisig. Removing the commit is not enough since history and forks keep it. Then rotate any related API keys, add secret scanning and pre-commit hooks, and load keys from environment variables or a secrets manager.

Show how to sign and verify a message with ethers.js.

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With ethers v6, a wallet signs using EIP-191 personal sign, and verifyMessage recovers the address. import { Wallet, verifyMessage } from 'ethers'; const wallet = Wallet.createRandom(); const message = 'Login nonce: 48213'; const signature = await wallet.signMessage(message); const recovered = verifyMessage(message, signature); console.log(recovered === wallet.address); // true This pattern powers Sign-In with Ethereum; the server issues a fresh nonce each time to stop replay.

Write a Python function that computes a Merkle root from a list of transactions.

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Hash leaves, then hash pairs until one remains, duplicating the last hash when a level is odd (Bitcoin style). import hashlib def h(b): return hashlib.sha256(b).digest() def merkle_root(txs): level = [h(t.encode()) for t in txs] while len(level) > 1: if len(level) % 2: level.append(level[-1]) level = [h(level[i] + level[i+1]) for i in range(0, len(level), 2)] return level[0].hex() print(merkle_root(['tx1', 'tx2', 'tx3']))

Describe a time you found a security problem in code and how you handled it.

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Situation: while reviewing a teammate's staking contract, I saw it sent tokens before updating the user's balance. Task: confirm the risk without blaming anyone. Action: I wrote a Foundry test with a malicious receiver that re-entered withdraw and drained extra funds, then shared it privately with a suggested fix using checks-effects-interactions and a reentrancy guard. Result: the fix merged before the audit, and we added reentrancy tests to our standard checklist.

What is a Merkle Patricia Trie and why does Ethereum use it?

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A Merkle Patricia Trie combines a Patricia (compressed prefix) trie with Merkle hashing. Ethereum uses it to store world state, account storage, transactions and receipts, with each root stored in the block header. It allows efficient insert, update and lookup by key, and any value can be proven against the root with a proof. Its weakness is large proof size, which is why Ethereum researchers plan to move to Verkle or binary hash trees for stateless clients.

What are zero-knowledge proofs, and how do zk-SNARKs and zk-STARKs differ?

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A zero-knowledge proof lets a prover convince a verifier that a statement is true without revealing why, for example proving a transaction is valid without exposing the details. zk-SNARKs have very small proofs and cheap verification, but many schemes need a trusted setup and rely on elliptic curves. zk-STARKs need no trusted setup, use hash-based cryptography considered post-quantum safe, but produce larger proofs. zk-rollups use them to prove batches of transactions to Ethereum.

What is signature malleability and how is it prevented?

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For any valid ECDSA signature (r, s), the value (r, n - s) is also valid for the same message. If code uses a signature as a unique ID, for example to mark it as used, an attacker can submit the flipped version and replay it. Ethereum's EIP-2 requires s to be in the lower half of the curve order for transactions. In contracts, use a library like OpenZeppelin ECDSA that enforces low s, and track nonces instead of signatures.

Design the custody setup for an exchange's hot and cold wallets.

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Keep a small float, around a few percent of assets, in hot wallets for withdrawals, secured by HSMs or MPC with rate limits, address allowlists and automated monitoring. Keep the rest in cold storage: air-gapped or multisig/MPC with keys held by different officers in different locations, plus time delays. Refill hot wallets through a manual, multi-approver process. Add proof of reserves, regular key ceremonies, insurance, and incident runbooks for freezing withdrawals if anomalies appear.

How do you verify a signed message inside a Solidity contract?

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Recompute the message hash, add the Ethereum signed message prefix, recover the signer and compare it. import {ECDSA} from '@openzeppelin/contracts/utils/cryptography/ECDSA.sol'; import {MessageHashUtils} from '@openzeppelin/contracts/utils/cryptography/MessageHashUtils.sol'; function verify(address signer, uint256 amount, uint256 nonce, bytes calldata sig) public view returns (bool) { bytes32 h = keccak256(abi.encode(address(this), block.chainid, amount, nonce)); bytes32 eth = MessageHashUtils.toEthSignedMessageHash(h); return ECDSA.recover(eth, sig) == signer; } Track used nonces to block replays.

What is a consensus mechanism and why does a blockchain need one?

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A consensus mechanism is the set of rules that lets thousands of independent nodes agree on one order of transactions and one current state, even when some nodes are faulty or malicious. Without it, two nodes could accept conflicting transactions and double spending would be possible. Common mechanisms are Proof of Work, Proof of Stake, Proof of Authority and BFT protocols like PBFT, Tendermint and Raft, each trading off decentralisation, speed and fault tolerance.

How does Proof of Work function?

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In Proof of Work, miners repeatedly change a nonce and hash the block header until the hash is below a difficulty target. Finding it takes huge computation, but anyone can verify it with one hash. The first miner to succeed broadcasts the block and earns the block reward plus fees. Nodes follow the chain with the most accumulated work. Bitcoin targets a block every ten minutes and adjusts difficulty every 2016 blocks to keep that pace.

Compare Proof of Work and Proof of Stake.

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Proof of Work secures the chain with electricity and mining hardware; it is battle-tested and simple but energy-hungry and tends to centralise in mining pools. Proof of Stake secures it with locked capital that can be slashed; it uses over 99 percent less energy, gives faster economic finality and lowers hardware needs. Its critics point to stake concentration in large staking providers and more complex protocol design. Ethereum moved from PoW to PoS in the Merge of September 2022.

What is a 51 percent attack?

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A 51 percent attack happens when one party controls a majority of mining power or stake. It can then reorder or exclude transactions, censor users and double spend by building a longer private chain that replaces recent blocks. It cannot steal coins from other addresses or forge signatures. Large chains like Bitcoin and Ethereum make it extremely expensive, but small PoW chains have suffered such attacks, which is why exchanges require many confirmations on them.

What is a Layer 2 and why is it needed?

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A Layer 2 is a network that executes transactions off the main chain and inherits security from it by posting data or proofs back to Layer 1. Ethereum Layer 1 handles only around 15 to 30 transactions per second at higher fees, so L2s batch thousands of transactions into one L1 submission, cutting fees to cents or less. Rollups are the main type. Users bridge assets from L1 to the L2 and can withdraw back.

What is Bitcoin halving and difficulty adjustment?

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Halving cuts the Bitcoin block reward in half every 210,000 blocks, roughly every four years, which caps total supply at 21 million BTC. After the April 2024 halving the reward is 3.125 BTC per block. Difficulty adjustment happens every 2016 blocks: if blocks came faster than ten minutes on average, difficulty rises, and if slower, it falls. Together they make issuance predictable regardless of how much mining power joins or leaves.

How does Ethereum's Proof of Stake work?

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Validators deposit 32 ETH (after the Pectra upgrade a validator can hold up to 2048 ETH) to join. Time is divided into 12-second slots and 32-slot epochs. In each slot a randomly chosen validator proposes a block and committees attest to it. The fork-choice rule LMD-GHOST picks the head, and Casper FFG finalises checkpoints when two-thirds of stake agrees. Honest validators earn rewards; offline ones lose small amounts, and provably malicious ones are slashed.

What is slashing in Proof of Stake?

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Slashing is a penalty that burns part of a validator's stake and forcibly exits it when it provably breaks protocol rules. On Ethereum the slashable offences are proposing two different blocks in the same slot and making conflicting or surrounding attestations. The penalty grows if many validators are slashed around the same time, which punishes coordinated attacks. Simply being offline is not slashing; it only causes small inactivity penalties. Running the same key on two machines is the most common cause.

What are Delegated Proof of Stake and Proof of Authority?

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In Delegated Proof of Stake, token holders vote for a small set of block producers who validate on their behalf, giving high throughput but more centralisation. In Proof of Authority, a fixed set of known, reputable validators sign blocks; their identity, not stake, is at risk. PoA, often via Clique, IBFT or QBFT, suits private and consortium EVM networks and some testnets, where participants are known and fast, predictable block times matter more than open participation.

What is the difference between optimistic rollups and zk-rollups?

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Optimistic rollups assume batches are valid and allow a challenge window, typically about seven days, during which anyone can submit a fraud proof; this makes them EVM-compatible and simpler but slows native withdrawals. Zk-rollups submit a validity proof with each batch, so L1 verifies correctness mathematically and withdrawals finalise once the proof is accepted. Zk proving is computationally heavy and EVM equivalence was harder, though zkEVMs have matured a lot by 2026.

How is a sidechain different from a rollup?

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A sidechain is an independent blockchain with its own validators and consensus, connected to the main chain by a bridge. Its security depends on its own validator set, so if they collude, funds can be stolen. A rollup posts its transaction data and state roots to Ethereum and relies on fraud or validity proofs, so it inherits Layer 1 security for correctness. Sidechains are usually cheaper and more flexible, but rollups give stronger guarantees.

What are state channels, such as the Lightning Network?

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A state channel locks funds in an on-chain contract or multisig between parties, who then exchange many signed off-chain updates and settle only the final state on-chain. Lightning on Bitcoin links channels into a network so payments route through intermediaries using hashed timelock contracts. Channels give instant, nearly free payments but need participants to be online or use watchtowers, and they lock liquidity, so they suit repeated payments between known parties rather than general smart contracts.

What is the difference between an execution client and a consensus client, and why does client diversity matter?

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Since the Merge, an Ethereum node runs two pieces of software. The execution client, such as Geth, Nethermind, Besu, Erigon or Reth, runs the EVM, holds state and handles transactions. The consensus client, such as Lighthouse, Prysm, Teku, Nimbus or Lodestar, runs Proof of Stake. They talk over the Engine API. Client diversity matters because a bug in a client used by over two-thirds of validators could finalise an invalid chain and cause mass slashing.

Your dApp's users are paying high gas fees on Ethereum mainnet. How would you choose a Layer 2?

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I would compare candidate rollups on security (proof system live, stage of decentralisation, upgrade keys), EVM compatibility so our Solidity code deploys unchanged, ecosystem fit (wallets, oracles, liquidity, bridges our users already use), fees and throughput, and withdrawal experience. I would deploy to testnets first, measure real costs and latency, and check that our dependencies such as Chainlink feeds exist there. Then migrate gradually, keeping L1 contracts for high-value settlement if needed.

A user is angry that withdrawing from an optimistic rollup to Ethereum takes seven days. How do you explain and solve it?

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Explain that the seven-day window lets anyone challenge a fraudulent batch; it protects their funds. For faster exits, offer third-party fast bridges or intent-based bridges where a liquidity provider pays them on L1 within minutes for a small fee and later claims the slow withdrawal. Also show the expected timeline clearly in the UI and send a notification when funds are claimable, because most anger comes from uncertainty, not the delay itself.

Your staking validator is missing attestations and losing rewards. How do you investigate?

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Check that both execution and consensus clients are synced and peering well, since low peer count or a lagging execution client causes missed duties. Look at system clock sync through NTP, CPU, memory and disk I/O, as slow disks often cause delays. Review client logs and metrics in Grafana, check for pending client upgrades before a fork, and verify network ports are open. Never start a second copy of the same validator key as a fix, because that causes slashing.

Write a simple Proof of Work loop in Python with a difficulty setting.

Show answer

The miner increments the nonce until the hash starts with the required number of zero hex digits. import hashlib def mine(data, difficulty=4): prefix = '0' * difficulty nonce = 0 while True: h = hashlib.sha256(f'{data}{nonce}'.encode()).hexdigest() if h.startswith(prefix): return nonce, h nonce += 1 print(mine('block-1: A pays B 5')) Each extra zero makes mining about 16 times harder, while verifying still needs one hash.

How do you deploy a contract to a Layer 2 testnet with Foundry?

Show answer

Set the RPC URL and a deployer key, then use forge create or a script, and verify the source on the explorer. export RPC_URL=https://sepolia.l2-example.org export PRIVATE_KEY=0x... # testnet key only forge create src/Counter.sol:Counter \ --rpc-url $RPC_URL --private-key $PRIVATE_KEY --broadcast # Scripted deploy with verification forge script script/Deploy.s.sol --rpc-url $RPC_URL \ --broadcast --verify Use a keystore via cast wallet import instead of raw keys for anything real.

How do you read the latest finalized block with ethers.js, and why use it?

Show answer

Pass the block tag to getBlock. The finalized tag returns the latest block that Proof of Stake has finalised, which cannot be reverted without slashing a third of stake. import { JsonRpcProvider } from 'ethers'; const provider = new JsonRpcProvider(process.env.RPC_URL); const latest = await provider.getBlock('latest'); const safe = await provider.getBlock('safe'); const finalized = await provider.getBlock('finalized'); console.log(latest.number - finalized.number); // usually about 64-96 Use finalized for deposits and accounting; use latest for UI updates.

Tell me about a technical decision you made where you had to balance trade-offs.

Show answer

Situation: our loyalty-points product needed on-chain points but users would not pay mainnet fees. Task: pick a chain. Action: I compared three rollups and a sidechain on fees, security model, wallet support and bridge maturity, built a quick benchmark, and wrote a one-page decision record with risks. Result: we chose a rollup with live fraud proofs, cut per-transaction cost by over 95 percent, and documented a migration path if our requirements changed.

Describe a time you disagreed with a teammate about architecture.

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Situation: a teammate wanted our indexer to read every event directly from the node at request time. Task: I believed it would not scale. Action: instead of arguing, I proposed a one-day spike, measured both approaches with realistic data, and shared numbers showing the database-backed indexer answered queries 40 times faster. Result: the team chose the indexer, my teammate led its reorg handling, and we agreed to settle future debates with quick experiments.

Explain Byzantine Fault Tolerance and the 3f+1 rule.

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Byzantine Fault Tolerance is the ability of a system to reach agreement even when some nodes lie or act arbitrarily. Classical BFT protocols such as PBFT, Tendermint and QBFT need at least 3f+1 nodes to tolerate f Byzantine nodes, because a quorum of 2f+1 votes guarantees that any two quorums overlap in at least one honest node. They give instant deterministic finality but involve heavy message passing, so they work best with tens to low hundreds of validators.

What did EIP-4844 (proto-danksharding) change?

Show answer

EIP-4844, shipped in the Dencun upgrade in March 2024, added blob-carrying transactions. Blobs are large data chunks of about 128 KB that rollups use to post batch data, priced in a separate fee market and pruned by nodes after roughly 18 days. Contracts cannot read blob contents, only their KZG commitments. This cut L2 data costs dramatically. Later upgrades raised blob capacity, and PeerDAS in the Fusaka upgrade lets nodes sample data instead of downloading every blob.

What is MEV and what is proposer-builder separation?

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MEV, maximal extractable value, is profit a block producer or searcher earns by including, excluding or reordering transactions, for example arbitrage, liquidations and sandwich attacks on swaps. On Ethereum, most validators use MEV-Boost, where specialised builders assemble profitable blocks and bid for them through relays, separating block building from proposing. This spreads MEV revenue to validators but concentrates building power. Enshrined PBS and inclusion lists are being researched to reduce censorship risk.

You are asked to design a cross-chain bridge. What are the main risks and design choices?

Show answer

Bridges hold large pooled funds, so they are top hacking targets. Main risks are compromised validator or multisig keys, faulty message verification, and replay of messages across chains. Prefer verification based on light clients or zk proofs over a small trusted multisig. Use rate limits, per-transfer caps, pausing, monitoring of mint-versus-lock balances, and a delay for large transfers. Get multiple audits, run a bug bounty, and consider established messaging protocols instead of building from scratch.

Four organisations are building a private EVM network. Which consensus would you pick: Raft or QBFT?

Show answer

Raft is crash fault tolerant: fast and simple, but it assumes no node lies, so it fits when all members trust each other's infrastructure. QBFT is Byzantine fault tolerant and tolerates f malicious nodes out of 3f+1, so with four validators it survives one bad actor and gives immediate finality. Since the organisations are separate businesses that may not fully trust each other, I would choose QBFT and run at least four validators, ideally more.

What is Ethereum and how is it different from Bitcoin?

Show answer

Ethereum is a programmable blockchain: besides transferring its native currency ETH, it runs smart contracts, which are programs stored on-chain that execute exactly as written. Bitcoin focuses on being secure, scarce digital money with a deliberately limited scripting language and the UTXO model. Ethereum uses an account model, Proof of Stake since 2022, about 12-second blocks, and a large ecosystem of tokens, DeFi, NFTs and Layer 2 rollups built on top.

What is the Ethereum Virtual Machine (EVM)?

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The EVM is the sandboxed, deterministic runtime that executes smart contract bytecode on every Ethereum node. It is a stack-based machine with 256-bit words, and each opcode has a gas cost, so every computation is metered and infinite loops are impossible. Because every node must get the same result, contracts cannot access the internet or randomness directly. Many chains and rollups are EVM-compatible, so the same Solidity contracts and tools work across them.

What is the difference between an externally owned account and a contract account?

Show answer

An externally owned account (EOA) is controlled by a private key, has a balance and nonce but no code, and is the only type that can start a transaction. A contract account has code and storage, is controlled by its code, and acts only when called. Since the Pectra upgrade's EIP-7702, an EOA can also delegate to contract code, so the line is blurring as smart accounts become common.

Explain gas, gas limit and gas price.

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Gas measures the computational work an operation needs; for example a simple ETH transfer uses 21,000 gas, and writing a new storage slot costs about 20,000. The gas limit is the maximum gas the sender allows a transaction to consume. The gas price, per unit, is what the sender pays. The fee equals gas used times the effective gas price. If execution exceeds the limit, it reverts but the used gas is still charged.

What happens when a transaction reverts?

Show answer

A revert undoes all state changes made during that transaction, including nested calls, as if it never ran. However, the transaction is still included in a block, the nonce is consumed, and the sender pays for the gas used up to the revert. Contracts revert with require, revert or custom errors, which return a reason the wallet can show. Since the Byzantium upgrade, unused gas is refunded on revert, unlike the old invalid opcode which consumed everything.

What are testnets and how do you get test ETH?

Show answer

Testnets are public networks that run the same software as mainnet but with valueless ETH, so developers can test deployments and dApps safely. Sepolia is the main testnet for application developers, and Hoodi replaced Holesky for validator and staking testing in 2025. Test ETH comes from faucets, which often require a small mainnet balance or a login to stop abuse. Many Layer 2s run their own Sepolia-based testnets bridged from Ethereum Sepolia.

What was the Merge?

Show answer

The Merge, completed in September 2022, switched Ethereum from Proof of Work to Proof of Stake by joining the original execution chain with the Beacon Chain, which had been running PoS since December 2020. Mining ended, energy use fell by more than 99.9 percent, and new ETH issuance dropped sharply. Block times became a fixed 12 seconds. Staked ETH withdrawals were enabled later in the Shapella upgrade of April 2023.

How do you get current fee data and send an EIP-1559 transaction with ethers.js?

Show answer

ethers v6 provides getFeeData, and sendTransaction fills in fields automatically, but you can set them explicitly. import { JsonRpcProvider, Wallet, parseEther } from 'ethers'; const provider = new JsonRpcProvider(process.env.RPC_URL); const wallet = new Wallet(process.env.PRIVATE_KEY, provider); const fee = await provider.getFeeData(); const tx = await wallet.sendTransaction({ to: '0xRecipient...', value: parseEther('0.01'), maxFeePerGas: fee.maxFeePerGas, maxPriorityFeePerGas: fee.maxPriorityFeePerGas }); const receipt = await tx.wait(); console.log(receipt.status);

Tell me about a blockchain project you have built.

Show answer

Pick one project and explain it with STAR. Situation: in my final year I wanted to learn Solidity end to end. Task: build a crowdfunding dApp. Action: I wrote the contract with goal and deadline logic and refunds, tested it with Foundry including fuzz tests, built a Next.js front-end with wagmi, and deployed to Sepolia. Result: 30 friends tested it, I fixed a refund bug they found, and the repo now has docs and a demo video.

How does the EIP-1559 fee model work?

Show answer

EIP-1559 splits the fee into a base fee and a priority fee. The protocol sets the base fee per block from demand: if the previous block was more than half full, it rises up to 12.5 percent, otherwise it falls. The base fee is burned. Users add a priority fee (tip) for the proposer and set maxFeePerGas as their cap. They pay base fee plus tip, never more than the cap, which makes fees more predictable.

What happens, step by step, when you send an Ethereum transaction?

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The wallet builds the transaction with nonce, to, value, data, gas limit and fee fields, signs it with the private key, and sends it to a node via eth_sendRawTransaction. The node validates and gossips it to other mempools. A block builder includes it, and the proposer publishes the block. Every node executes it in the EVM, updating state and producing a receipt with status, gas used and logs. After about two epochs the block is finalised.

Explain storage, memory, stack and calldata in the EVM.

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Storage is persistent key-value data of 32-byte slots per contract; it is the most expensive, and writes to new slots cost about 20,000 gas. Memory is a temporary byte array cleared after each call, cheap but growing in cost quadratically. The stack holds up to 1024 words for opcode operands. Calldata is the read-only input data of the transaction, the cheapest place to read external function arguments from. Choosing the right location is a key gas optimisation.

What are the ABI and function selectors?

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The ABI (Application Binary Interface) is the JSON description of a contract's functions, events and errors, and it defines how arguments are encoded. A function selector is the first four bytes of the Keccak-256 hash of the function signature, for example transfer(address,uint256) gives 0xa9059cbb. Calldata is the selector followed by ABI-encoded arguments in 32-byte words. Wallets, ethers.js and explorers use the ABI to encode calls and decode results and events.

What are events and logs in Ethereum?

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Events are declared in Solidity and emitted with emit; the EVM writes them as logs in the transaction receipt. Each log has the contract address, up to three indexed topics plus the event signature hash, and non-indexed data. Logs are much cheaper than storage but contracts cannot read them. Front-ends and indexers subscribe to events to update the UI, track token transfers and build history, so emitting clear events for every state change is good practice.

What is the difference between CREATE and CREATE2?

Show answer

CREATE computes a new contract's address from the deployer's address and nonce, so the address depends on deployment order. CREATE2 computes it from the deployer address, a chosen 32-byte salt and the hash of the init code, so the address is known before deployment and is the same wherever those inputs match. CREATE2 enables counterfactual wallets, deterministic factory deployments and same-address deployments across chains, but redeploying at the same address needs care.

A transaction keeps failing with an out of gas error. How do you fix it?

Show answer

First see whether the gas limit was too low or the function is genuinely unbounded. Run eth_estimateGas or simulate with cast run or Tenderly to see actual gas used and where it fails. If estimation is fine, raise the limit with a buffer of 10 to 20 percent. If the function loops over a growing array, redesign it with pagination or pull-based patterns, because the block gas limit will eventually make it impossible to call.

You need the same contract at the same address on five chains. How do you do it?

Show answer

Use CREATE2 through a deterministic deployment factory that already exists at the same address on all those chains. Keep the init code identical: same compiler version, optimiser settings, constructor arguments and metadata. Pick a salt, compute the expected address off-chain, then deploy on each chain. Foundry scripts support this by passing a salt. Avoid constructor arguments that differ per chain, or move chain-specific settings into an initialiser restricted to the deployer.

Your dApp needs a full history of user transactions and fast queries. Reading events from the node is too slow. What do you do?

Show answer

Build or use an indexer. An indexer listens to contract events from a start block, transforms them and stores them in a database like PostgreSQL, so the front-end runs normal fast queries. Options are a subgraph on The Graph, hosted indexing services, or a custom service with ethers and a queue. It must handle reorgs by tracking block hashes, support backfilling, and expose an API. Design events up front with indexed fields that queries need.

How do you read a contract's storage slot directly with Foundry's cast?

Show answer

Use cast storage with the contract address and slot number. For mappings, compute the slot as keccak256(key . slot). # Read slot 0 (often the first state variable) cast storage 0xContractAddress 0 --rpc-url $RPC_URL # balanceOf mapping stored at slot 2, key = user address cast index address 0xUserAddress 2 cast storage 0xContractAddress <slot-from-above> --rpc-url $RPC_URL This shows that private variables are not secret: anyone can read storage.

How do you compute a function selector and decode calldata with cast?

Show answer

cast handles ABI work from the command line. # Function selector cast sig 'transfer(address,uint256)' # 0xa9059cbb # Encode calldata cast calldata 'transfer(address,uint256)' 0xAbc... 1000000 # Decode calldata from a transaction cast 4byte-decode 0xa9059cbb000000... # Decode with a known signature cast decode-calldata 'transfer(address,uint256)' 0xa9059cbb... This is useful when checking what a wallet is really asking you to sign.

What is the difference between call, delegatecall and staticcall?

Show answer

call runs the target contract's code in the target's own context, with its own storage, and msg.sender becomes the caller contract. delegatecall runs the target's code in the caller's context, using the caller's storage, balance and original msg.sender; this is how proxies and libraries work, and storage layouts must match. staticcall is like call but reverts on any state change, used for view functions. Delegatecall to untrusted code is extremely dangerous.

What transaction types exist on Ethereum today?

Show answer

Type 0 is the legacy transaction with a single gasPrice. Type 1, from EIP-2930, adds an access list. Type 2, from EIP-1559, uses maxFeePerGas and maxPriorityFeePerGas and is the default in wallets. Type 3, from EIP-4844, carries blobs for rollup data. Type 4, from EIP-7702 in the Pectra upgrade of May 2025, lets an EOA set code delegation through a signed authorisation list, enabling batching and sponsored gas for normal wallets.

What is account abstraction, and how do ERC-4337 and EIP-7702 differ?

Show answer

Account abstraction makes accounts programmable, so wallets can support batching, gas paid in tokens or by sponsors, passkeys, spending limits and social recovery. ERC-4337 does this without protocol changes: users send UserOperations to an alternate mempool, bundlers submit them to an EntryPoint contract, and paymasters can sponsor gas. EIP-7702 is a protocol change that lets an existing EOA point to smart account code, so users keep their address while gaining these features.

100

A simulated eth_call succeeds but the real transaction reverts. What could cause this?

Show answer

The state can change between simulation and inclusion: price moved beyond slippage limits, a deadline passed, another user took the last item, or a bot front-ran the trade. Other causes are a different msg.sender or value in the simulation, a gas limit too low for the real path, block-dependent logic like block.timestamp, or a different block tag. Debug by tracing the failed transaction with cast run or a tracer and comparing inputs and state.

101

Describe a production incident you handled under pressure.

Show answer

Situation: after a release, our token-claim page started failing for some users while traffic was peaking. Task: as on-call lead I had to restore service quickly. Action: I paused the claim contract through our multisig, traced failing transactions, and found a front-end bug sending an outdated Merkle proof format. We hot-fixed the UI, tested on a fork, and unpaused. Result: downtime was 40 minutes, no funds were at risk, and we added an end-to-end check against the deployed root.

102

What is Solidity?

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Solidity is a statically typed, contract-oriented, curly-brace language for writing smart contracts on Ethereum and other EVM chains. Its syntax resembles JavaScript and C++, and it compiles to EVM bytecode plus an ABI. It supports inheritance, libraries, user-defined types, custom errors and events. Version 0.8.x is current, with checked arithmetic by default. Developers usually write it with Foundry or Hardhat and reuse audited building blocks from OpenZeppelin Contracts.

103

What are value types and reference types in Solidity, and what are data locations?

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Value types such as uint, int, bool, address, bytes32 and enums are copied when assigned or passed. Reference types such as arrays, structs, strings and mappings point to data, so you must state a data location: storage for persistent contract state, memory for temporary data inside a function, or calldata for read-only external inputs. Assigning storage to a local storage pointer modifies state, while copying to memory creates an independent copy.

104

Explain public, external, internal and private visibility.

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public functions can be called externally and internally, and public state variables get an automatic getter. external functions can only be called from outside, or via this.f(), and read arguments directly from calldata. internal functions and variables are available to the contract and contracts that inherit it. private is limited to the defining contract only. Note that private only restricts other contracts; the data is still fully visible on the blockchain to anyone.

105

What is the difference between view, pure and payable functions?

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A view function reads state but does not modify it, so it is free when called off-chain via eth_call. A pure function neither reads nor modifies state and works only on its inputs, for example a math helper. A payable function can receive ETH along with the call; non-payable functions revert if ETH is sent. View and pure functions still cost gas when called inside a state-changing transaction.

106

What are function modifiers in Solidity?

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Modifiers are reusable pieces of code that run before or after a function body, placed where the underscore symbol appears. They are mainly used for checks such as onlyOwner, whenNotPaused or nonReentrant. For example, modifier onlyOwner() { require(msg.sender == owner, 'not owner'); _; } blocks everyone except the owner. Keep modifiers short and check-only; complex logic inside modifiers hides behaviour and increases bytecode size because the code is inlined into every function that uses it.

107

What is the difference between constant and immutable?

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constant variables must be assigned a compile-time value and are inlined into the bytecode, for example uint256 constant MAX = 1000. immutable variables are assigned once in the constructor and then embedded in the deployed code, for example an owner address or token address chosen at deployment. Neither uses a storage slot, so reading them is far cheaper than reading normal state. Use them for anything that never changes after deployment.

108

Write a simple ERC-20 token using OpenZeppelin.

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Inherit the audited ERC20 implementation and mint the initial supply in the constructor. // SPDX-License-Identifier: MIT pragma solidity ^0.8.24; import {ERC20} from '@openzeppelin/contracts/token/ERC20/ERC20.sol'; import {Ownable} from '@openzeppelin/contracts/access/Ownable.sol'; contract CampusToken is ERC20, Ownable { constructor() ERC20('Campus Token', 'CAMP') Ownable(msg.sender) { _mint(msg.sender, 1_000_000 * 10 ** decimals()); } function mint(address to, uint256 amount) external onlyOwner { _mint(to, amount); } }

109

Tell me about a time you learnt a new technology quickly to meet a deadline.

Show answer

Situation: in my internship I was asked to add NFT minting to a demo in ten days, and I had never written Solidity. Task: deliver a working, tested contract. Action: I spent two days on Solidity basics and OpenZeppelin docs, built the ERC-721 contract, asked a senior to review my approach early, and wrote Foundry tests for every function. Result: the demo shipped on time on Sepolia, and the review caught only minor issues.

110

How are mappings stored, and why can you not iterate over them?

Show answer

A mapping does not store keys. The value for key k in a mapping declared at slot p lives at storage slot keccak256(k . p). Every possible key implicitly exists with a default zero value, so the contract cannot know which keys were set or how many. To iterate, keep a separate array of keys or use OpenZeppelin EnumerableSet or EnumerableMap, accepting extra gas, or rebuild the list off-chain from emitted events.

111

What is the difference between require, revert, assert and custom errors?

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require(condition, message) validates inputs and conditions, reverting with a message if false. revert stops execution explicitly and is often used with custom errors, such as error InsufficientBalance(uint256 need, uint256 have); revert InsufficientBalance(x, y); which are cheaper than strings and carry data. assert checks invariants that should never fail; a failure signals a bug and raises a Panic. Since Solidity 0.8.26 you can also pass custom errors to require.

112

How does inheritance work in Solidity, including virtual and override?

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A contract inherits with is, for example contract Token is ERC20, Ownable. Parent functions that may be changed must be marked virtual, and the child must use override; with several parents you list them, as in override(A, B). Solidity uses C3 linearisation, so the order of the parent list matters, from most base to most derived, and super calls the next contract in that order. Constructors of parents run first in linearised order.

113

Compare interfaces, abstract contracts and libraries.

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An interface only declares external functions, events and errors, with no state or implementation; it is used to call other contracts, like IERC20. An abstract contract can have state and implemented functions but leaves at least one unimplemented or is marked abstract, so it cannot be deployed alone. A library holds reusable functions without state; internal library functions are inlined, while external ones are deployed once and called via delegatecall. Libraries can attach to types with using for.

114

What is the difference between receive and fallback functions?

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receive() external payable runs when a contract receives plain ETH with empty calldata, for example a transfer from a wallet. fallback() runs when calldata matches no function, or when ETH arrives with data and no receive exists; it must be payable to accept ETH. Proxies use fallback to delegatecall all unknown calls to the implementation. Keep both simple, since a reverting or gas-heavy receive can break senders that use transfer with its 2300 gas stipend.

115

How does arithmetic overflow work in Solidity 0.8, and when do you use unchecked?

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Since 0.8.0, arithmetic is checked by default: overflow or underflow reverts with a Panic, so SafeMath is no longer needed. Checks cost a little gas. Wrapping code in an unchecked block disables them, which is safe when you have proven the value cannot overflow, such as a loop counter bounded by an array length, or subtracting after a require that a is at least b. Using unchecked carelessly reintroduces the old overflow bugs.

116

The business wants to change contract logic after launch. How do you plan for it?

Show answer

Decide early whether upgradeability is needed, because it adds trust and storage risks. Options are a proxy (UUPS or Transparent) controlled by a multisig with a timelock, a parameter-driven design where only settings like fees change through governed setters, or a migration plan to a new immutable version. Whatever you choose, document it for users, protect upgrade rights, keep storage layout append-only, and test each upgrade on a fork before executing.

117

A function that distributes rewards loops over all users and is getting too expensive. How do you fix it?

Show answer

Unbounded loops eventually exceed the block gas limit. Switch to a pull-based accounting model: maintain a global reward-per-token accumulator that grows when rewards are added, and store each user's last checkpoint. Users claim with reward equals balance times (current accumulator minus their checkpoint). This makes distribution O(1) for any number of users. Alternatively, use a Merkle distributor where rewards are computed off-chain and users claim with proofs.

118

You need to send ETH to hundreds of winners. Should you push payments or let them pull?

Show answer

Prefer pull payments. If a contract pushes ETH in a loop, one recipient that reverts, perhaps a contract without receive, can block everyone, and the loop may exceed the gas limit. With pull payments, the contract records each winner's amount in a mapping and winners call withdraw, which zeroes their balance before sending. This isolates failures and limits reentrancy risk. Push only for small fixed lists using call with a checked result.

119

Write a Foundry unit test and a fuzz test for a contract.

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Foundry tests are Solidity contracts; functions starting with test run as tests, and parameters make them fuzz tests. import {Test} from 'forge-std/Test.sol'; import {Vault} from '../src/Vault.sol'; contract VaultTest is Test { Vault v; function setUp() public { v = new Vault(); } function testDeposit() public { v.deposit{value: 1 ether}(); assertEq(v.balanceOf(address(this)), 1 ether); } function testFuzzDeposit(uint96 amt) public { vm.assume(amt > 0); vm.deal(address(this), amt); v.deposit{value: amt}(); assertEq(address(v).balance, amt); } } Run with forge test -vvv.

120

Write a simple escrow contract where a buyer deposits and releases payment to a seller.

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pragma solidity ^0.8.24; contract Escrow { address public immutable buyer; address payable public immutable seller; bool public released; constructor(address payable _seller) payable { buyer = msg.sender; seller = _seller; } function release() external { require(msg.sender == buyer, 'only buyer'); require(!released, 'done'); released = true; (bool ok, ) = seller.call{value: address(this).balance}(''); require(ok, 'transfer failed'); } } State is updated before the external call to follow checks-effects-interactions.

121

Write a basic voting contract using a struct, mapping and event.

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pragma solidity ^0.8.24; contract Voting { struct Proposal { string name; uint256 votes; } Proposal[] public proposals; mapping(address => bool) public hasVoted; event Voted(address indexed voter, uint256 indexed id); constructor(string[] memory names) { for (uint256 i; i < names.length; i++) proposals.push(Proposal(names[i], 0)); } function vote(uint256 id) external { require(!hasVoted[msg.sender], 'already voted'); hasVoted[msg.sender] = true; proposals[id].votes++; emit Voted(msg.sender, id); } } An out-of-range id reverts automatically.

122

How did you handle a time you received critical code review feedback?

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Situation: a senior reviewer rejected my pull request for a staking feature, saying the storage layout would break future upgrades. Task: fix it and learn. Action: I thanked him, asked him to explain the risk, read about EIP-1967 and storage gaps, rewrote the contract with namespaced storage, and added a storage-layout check to CI. Result: the PR merged, and that check later caught a teammate's layout change before an upgrade.

123

How have you mentored junior developers on a smart contract team?

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Situation: our team hired three freshers who knew Solidity syntax but not security. Task: get them productive safely. Action: I created a two-week plan with Ethernaut challenges, pair-programmed their first features, and made them write tests before code. In reviews I explained why, not just what, and kept a shared checklist of common bugs. Result: within two months they shipped features independently, and one found a rounding bug during an internal review.

124

How do upgradeable contracts work, and how do Transparent and UUPS proxies differ?

Solidity ProgrammingHard

Show answer

A proxy holds the state and delegates every call to an implementation contract whose address is stored in a standard EIP-1967 slot; upgrading means pointing to a new implementation. In the Transparent pattern, upgrade logic lives in the proxy and an admin cannot call implementation functions, which avoids selector clashes. In UUPS, upgrade logic lives in the implementation, making proxies cheaper, but a faulty upgrade can brick it. Both use initialisers instead of constructors and require storage-layout compatibility.

125

What is storage packing, and what are other key gas optimisations?

Solidity ProgrammingHard

Show answer

The EVM stores data in 32-byte slots, so consecutive small variables like uint128, uint64, address and bool can share a slot if declared next to each other, saving expensive SSTOREs. Other optimisations: use calldata for external arrays, cache storage reads in local variables, use immutable and constant, prefer custom errors over strings, avoid unbounded loops, use events instead of storing history, and use unchecked for counters that cannot overflow. Always measure with forge snapshot.

126

Your contract fails to deploy because it exceeds the 24 KB size limit. What do you do?

Solidity ProgrammingHard

Show answer

The EIP-170 limit is 24,576 bytes of runtime code. First enable the optimiser and try a lower runs value or via-IR. Replace revert strings with custom errors, move repeated logic out of modifiers into internal functions, and remove unused code. If still too big, split logic into separate contracts or external libraries, or use a modular pattern like the Diamond (EIP-2535). Splitting also often improves clarity and auditability.

127

What is the checks-effects-interactions pattern?

Show answer

It is an ordering rule for functions. First do checks: validate inputs, permissions and balances with require. Then apply effects: update all state variables, such as reducing the user's balance. Only then do interactions: external calls, token transfers or ETH sends. Because state is already updated, a re-entering call sees the new state and cannot repeat the action. Combine it with a nonReentrant modifier for defence in depth, especially in functions calling unknown contracts.

128

How did integer overflow bugs happen before Solidity 0.8?

Show answer

Before 0.8, arithmetic wrapped silently: a uint8 at 255 plus 1 became 0, and 0 minus 1 became the maximum value. Attackers exploited this, for example by making a balance underflow to a huge number or an overflowed multiplication pass a check, as in the 2018 batchOverflow token bug. Developers used the SafeMath library to revert on overflow. Since 0.8 the compiler checks automatically, except inside unchecked blocks and in inline assembly.

129

Why should you not use tx.origin for authorisation?

Show answer

tx.origin is the EOA that started the transaction, while msg.sender is the immediate caller. If a contract checks require(tx.origin == owner), a phishing contract can trick the owner into calling it, and that contract then calls the victim; tx.origin is still the owner, so the check passes and funds can be stolen. Always use msg.sender for access control. tx.origin also breaks with smart contract wallets and account abstraction.

130

A user approved unlimited token spending to a scam site. What should they do?

Show answer

Revoke the approval immediately using a token approval checker or by calling approve(spender, 0) from the wallet, because the malicious contract can pull tokens any time. If a drain has started, move remaining assets to a fresh wallet. Check for other suspicious approvals and NFT setApprovalForAll permissions. Going forward, approve exact amounts, read wallet prompts carefully, use a separate hot wallet for new dApps, and prefer wallets that simulate transactions before signing.

131

How do you run Slither on a Foundry project and focus on important findings?

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Install it with pip and run it from the project root; it detects Foundry automatically. pip install slither-analyzer slither . # Only medium and high impact issues slither . --exclude-low --exclude-informational # Exclude dependencies slither . --filter-paths 'lib|test' # Print a summary of contracts and functions slither . --print human-summary Triage each finding: fix real bugs and record why false positives are safe.

132

What is a reentrancy attack?

Show answer

Reentrancy happens when a contract makes an external call, such as sending ETH, before updating its own state, and the receiver calls back into the contract to repeat the action. For example, a withdraw that sends funds and then sets the balance to zero can be re-entered to withdraw many times. The 2016 DAO hack drained millions this way. Prevent it with checks-effects-interactions, reentrancy guards, and pull payments.

133

What are front-running and sandwich attacks, and how do you mitigate them?

Show answer

Pending transactions are public, so bots can copy or outbid them. Front-running is placing a transaction ahead of a victim's, for example buying before a large purchase. A sandwich attack buys before the victim's swap and sells right after, profiting from the price impact. Mitigations include slippage limits and deadlines, commit-reveal schemes, batch auctions, private transaction relays, and design that does not reward ordering, such as using time-weighted prices.

134

What are common access control vulnerabilities?

Show answer

Typical bugs include missing onlyOwner or role checks on sensitive functions such as mint, withdraw or setOracle, initialiser functions in upgradeable contracts that anyone can call, misconfigured roles, and a single EOA owning critical powers. Use OpenZeppelin Ownable2Step or AccessControl, give each role the minimum rights, protect admin actions with a multisig and timelock, and write tests confirming that every privileged function reverts for unauthorised callers.

135

How can a smart contract suffer denial of service?

Show answer

Common causes: loops over arrays that grow with users until they exceed the block gas limit; push payments in a loop where one reverting recipient blocks everyone; relying on an external call that an attacker can make fail; and griefing by filling storage cheaply. For example, an auction that refunds the previous bidder will freeze if that bidder is a contract that reverts. Use pull payments, bounded loops, pagination and failure isolation.

136

Why is on-chain randomness insecure, and what should you use instead?

Show answer

Values like block.timestamp, blockhash or block.prevrandao are visible to or partly controlled by block producers, and any contract can read them in the same transaction and only proceed when the outcome is favourable. So lotteries and NFT trait reveals based on them can be gamed. Use a verifiable random function such as Chainlink VRF, which delivers randomness with a cryptographic proof in a later transaction, or a commit-reveal scheme for simpler cases.

137

Why should you check return values of low-level calls and use SafeERC20?

Show answer

Low-level call, send and delegatecall return false instead of reverting on failure, so ignoring the result means the contract continues as if the transfer succeeded. Similarly, some ERC-20 tokens do not return a bool from transfer, and others return false instead of reverting. OpenZeppelin's SafeERC20 wraps transfer, transferFrom and approve to handle both cases and revert on failure. Always write (bool ok, ) = to.call{value: x}(''); require(ok).

138

What is a signature replay attack and how do you prevent it?

Show answer

If a contract accepts a signed message to perform an action, an attacker can resubmit the same signature to repeat it, or replay it on another chain or another contract using the same code. Prevent it by including a per-user nonce that increments on use, a deadline, the chain ID and the verifying contract address in the signed data. EIP-712 typed data with a domain separator gives a standard way to include these fields.

139

What does a smart contract audit process look like?

Show answer

It starts with scoping: fixed commit, documentation, threat model and test coverage. Auditors run automated tools like Slither and fuzzers, then do manual line-by-line review, checking business logic, access control, economic attacks and integrations. They write findings rated critical, high, medium, low and informational with fixes. The team fixes issues, auditors review the fixes, and a final report is published. Audits reduce risk but do not guarantee safety, so add bug bounties and monitoring.

140

Which tools do you use for static analysis and fuzzing of smart contracts?

Show answer

Slither performs static analysis and quickly flags reentrancy, unchecked calls, shadowing and more. Aderyn is another fast static analyser. Foundry provides fuzz and invariant testing, where random sequences of calls try to break properties you define. Echidna and Medusa are property-based fuzzers. Mythril and Halmos use symbolic execution, and Certora offers formal verification. Combining them with manual review and high test coverage catches far more than any single tool.

141

An auditor flags that a single owner key can mint tokens and change fees. How do you address it?

Show answer

This is centralisation risk: if the key is compromised or misused, users lose funds. Move ownership to a multisig with signers on separate devices, add a timelock so users can see and react to changes, set hard-coded bounds such as a maximum fee, and remove powers that are not needed, like unlimited minting. Use role-based access instead of one all-powerful owner, and document the remaining admin powers publicly.

142

Your team is preparing a contract for an external audit. What do you do beforehand?

Show answer

Freeze the code at a commit and stop adding features. Write clear documentation: architecture, roles, trust assumptions, invariants and known risks. Reach high test coverage including fuzz and invariant tests, and run Slither and fix its findings. Remove dead code, add NatSpec comments, and make the build reproducible. Share a scope list and a walkthrough call. A clean, well-documented codebase lets auditors spend their limited time on deep logic bugs.

143

Show a contract vulnerable to reentrancy and a fixed version.

Show answer

Vulnerable: the balance is cleared after sending. function withdraw() external { uint256 bal = balances[msg.sender]; (bool ok, ) = msg.sender.call{value: bal}(''); require(ok); balances[msg.sender] = 0; } Fixed: update state first and add a guard. import {ReentrancyGuard} from '@openzeppelin/contracts/utils/ReentrancyGuard.sol'; function withdraw() external nonReentrant { uint256 bal = balances[msg.sender]; require(bal > 0, 'nothing'); balances[msg.sender] = 0; (bool ok, ) = msg.sender.call{value: bal}(''); require(ok, 'send failed'); }

144

Tell me about a bug you missed and what you learned from it.

Show answer

Situation: I shipped a rewards contract where a rounding error let users claim slightly more than earned when claiming very often. Task: fix it and prevent repeats. Action: I owned the mistake in the team call, wrote a failing test reproducing it, changed the division order to round down in the protocol's favour, and added fuzz tests on claim frequency. Result: losses were tiny, and I now always fuzz any maths involving division.

145

How do flash loans enable oracle manipulation attacks?

Show answer

A flash loan lets anyone borrow huge amounts without collateral if it is repaid in the same transaction. An attacker borrows, makes a large trade in a thin DEX pool to move its spot price, then uses a protocol that reads that pool as its price oracle, for example borrowing too much against inflated collateral, and finally reverses the trade and repays. Defend with decentralised oracles like Chainlink, TWAPs, multiple sources and sanity bounds.

146

What are storage collision and uninitialised implementation risks in proxies?

Show answer

With delegatecall, the implementation's code writes to the proxy's storage. If the proxy stores its admin or implementation address in a normal slot, implementation variables can overwrite it, so EIP-1967 uses pseudo-random slots. New implementation versions must only append variables, or use namespaced storage (ERC-7201). Implementations must also disable initialisers in their constructor; otherwise an attacker can initialise the implementation directly, take ownership and, in UUPS, potentially destroy or hijack it.

147

What are cross-function and read-only reentrancy?

Show answer

Cross-function reentrancy happens when an attacker re-enters a different function that shares state with the one making the external call, so a guard on only one function does not help. Read-only reentrancy happens when a view function, such as a pool's price getter, returns inconsistent state mid-execution, and another protocol reads it during the callback and misprices assets. Mitigate by guarding all state-sharing functions, updating state before calls, and checking a reentrancy lock in view functions other protocols rely on.

148

Your protocol is being exploited right now. What do you do?

Show answer

Act fast with a prepared runbook. Pause affected contracts through the guardian or multisig, and disable front-end actions. Gather the core team and security partners in a war room. Trace the attack transactions to understand the bug and estimate losses, and check whether a white-hat rescue of remaining funds is needed. Communicate publicly with facts and no speculation, contact exchanges and bridges to flag stolen funds, then fix, audit the fix and publish a post-mortem.

149

A lending protocol reads collateral prices from one DEX pool's current reserves. What is the risk and fix?

Show answer

Spot prices from a single pool can be moved within one transaction by a flash-loaned swap, letting an attacker inflate collateral value and borrow far more than it is worth, or trigger unfair liquidations. Replace it with a robust oracle such as Chainlink price feeds, or a long-window TWAP from deep pools, check staleness and deviation between sources, cap borrow amounts per block, and add circuit breakers for extreme price moves.

150

Write a Foundry invariant test for a token's total supply.

Show answer

Invariant tests call random functions on target contracts and check a property after every call. import {Test} from 'forge-std/Test.sol'; import {MyToken} from '../src/MyToken.sol'; import {Handler} from './Handler.sol'; contract SupplyInvariant is Test { MyToken token; Handler handler; function setUp() public { token = new MyToken(); handler = new Handler(token); targetContract(address(handler)); } function invariant_supplyMatchesBalances() public view { assertEq(token.totalSupply(), handler.sumOfBalances()); } } The handler bounds inputs and tracks actors so calls stay realistic.

151

Management wants to launch before the audit is complete. How do you handle it?

Show answer

Situation: marketing had fixed a launch date, but the audit had two open high-severity findings. Task: protect users without blocking the business. Action: I explained the risk in money terms, proposed a guarded launch with a low deposit cap, pausable contracts and only the audited modules enabled, and shared a timeline for the fixes. Result: leadership accepted, we launched on time with caps, and raised limits two weeks later after fixes were verified.

152

What is DeFi?

Show answer

DeFi, decentralised finance, means financial services such as trading, lending, borrowing, stablecoins and derivatives built as smart contracts on public blockchains. Anyone with a wallet can use them without a bank account or approval, contracts are transparent, and protocols can be combined like building blocks. Examples are automated market maker exchanges, overcollateralised lending markets and liquid staking. The risks are smart contract bugs, oracle failures, volatile collateral and unclear regulation.

153

Compare ERC-20, ERC-721 and ERC-1155 tokens.

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ERC-20 defines fungible tokens where every unit is identical, with functions like transfer, approve, transferFrom and balanceOf; used for currencies, stablecoins and governance tokens. ERC-721 defines non-fungible tokens where each tokenId is unique, used for NFTs like art, tickets or certificates. ERC-1155 is a multi-token standard where one contract manages many token types, fungible or not, with batch transfers; it is popular in games where items have quantities.

154

What types of stablecoins exist?

Show answer

Fiat-backed stablecoins hold cash and short-term government bonds off-chain, one dollar per token, and depend on the issuer's audits and redemptions. Crypto-collateralised stablecoins are minted against overcollateralised crypto in smart contracts, such as vaults requiring 150 percent collateral. Algorithmic stablecoins try to hold the peg with supply rules and incentives; several have collapsed, the most famous in 2022. Choose based on trust model, liquidity, transparency and regulatory status.

155

Write a basic ERC-721 NFT contract using OpenZeppelin.

Show answer

// SPDX-License-Identifier: MIT pragma solidity ^0.8.24; import {ERC721} from '@openzeppelin/contracts/token/ERC721/ERC721.sol'; import {Ownable} from '@openzeppelin/contracts/access/Ownable.sol'; contract CertificateNFT is ERC721, Ownable { uint256 public nextId; string private baseURI; constructor(string memory uri) ERC721('Course Cert', 'CERT') Ownable(msg.sender) { baseURI = uri; } function mint(address to) external onlyOwner { _safeMint(to, nextId++); } function _baseURI() internal view override returns (string memory) { return baseURI; } }

156

How do you approve and transfer ERC-20 tokens from a script with ethers.js?

Show answer

import { Contract, parseUnits } from 'ethers'; const abi = [ 'function approve(address spender, uint256 amount) returns (bool)', 'function decimals() view returns (uint8)', 'function allowance(address owner, address spender) view returns (uint256)' ]; const token = new Contract(TOKEN_ADDRESS, abi, signer); const dec = await token.decimals(); const amount = parseUnits('25', dec); const tx = await token.approve(SPENDER, amount); await tx.wait(); console.log(await token.allowance(signer.address, SPENDER)); The spender contract then calls transferFrom to pull exactly that amount.

157

How should NFT metadata and tokenURI be structured?

Show answer

tokenURI(id) returns a URI, usually ipfs://<CID>/<id>.json, pointing to JSON that marketplaces read: { "name": "Course Certificate #12", "description": "Completed Blockchain Developer course", "image": "ipfs://bafy.../12.png", "attributes": [ { "trait_type": "Grade", "value": "A" }, { "trait_type": "Year", "value": 2026 } ] } Store images and JSON on IPFS or Arweave rather than a normal server, so metadata cannot silently change or disappear.

158

A retail brand wants to give customers loyalty points as blockchain tokens. How would you design it?

Show answer

Keep it simple for non-crypto users. Use a low-fee Layer 2 or a permissioned chain, and create wallets automatically with account abstraction or embedded wallets linked to phone login, so customers never handle seed phrases. Sponsor gas through a paymaster. Make points a non-transferable or restricted ERC-20 if resale is not intended, to avoid VDA tax and securities issues. Keep customer data off-chain and show points in the brand's normal app.

159

Why do you want to work in DeFi, given its risks and volatility?

Show answer

I am drawn to DeFi because it turns financial products into open, auditable code, and that combination of finance and engineering motivates me. I understand the risks: hacks, market crashes and regulatory change. That is why I have focused on security, testing and risk thinking, not just writing contracts. I see volatility as a reason to build carefully. I want to work on a team that values audits, monitoring and user protection.

160

How does a constant product automated market maker work, and what is impermanent loss?

Show answer

A constant product AMM holds two tokens with reserves x and y and keeps x times y equal to k. A trader adds one token and removes the other so that k stays constant after fees, which makes price equal to the reserve ratio and rise with trade size. Liquidity providers earn fees, but when prices move, arbitrage rebalances the pool, leaving them worse off than simply holding; that gap is impermanent loss.

161

Calculate the output of a swap in a constant product pool with a 0.3 percent fee.

Show answer

Formula: amountOut = (amountIn x 997 x reserveOut) / (reserveIn x 1000 + amountIn x 997). Example: reserves 1,000 ETH and 3,000,000 USDC, swap 10 ETH. amountInWithFee = 10 x 997 = 9,970 numerator = 9,970 x 3,000,000 = 29,910,000,000 denominator = 1,000 x 1000 + 9,970 = 1,009,970 amountOut = 29,615 USDC (approx) The spot price is 3,000, but the trader gets about 2,961.5 per ETH because of fee and price impact.

162

How do you calculate a borrower's health factor in a lending protocol?

Show answer

Health factor = (sum of collateral value x liquidation threshold) / total debt value. Below 1, the position can be liquidated. Example: collateral 10 ETH at 3,000 dollars = 30,000 dollars, liquidation threshold 80 percent, debt 18,000 dollars of stablecoins. HF = (30,000 x 0.80) / 18,000 = 1.33. If ETH falls to 2,250, collateral is 22,500, so HF = 18,000 / 18,000 = 1.0 and liquidation begins. Users raise HF by repaying debt or adding collateral.

163

How do you read a Chainlink price feed in Solidity safely?

Show answer

import {AggregatorV3Interface} from '@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV3Interface.sol'; AggregatorV3Interface public feed; uint256 constant MAX_DELAY = 1 hours; function getPrice() public view returns (uint256) { (, int256 answer, , uint256 updatedAt, ) = feed.latestRoundData(); require(answer > 0, 'bad price'); require(block.timestamp - updatedAt <= MAX_DELAY, 'stale price'); return uint256(answer); // scale using feed.decimals(), often 8 } Always check staleness, sign and decimals; on L2s also check the sequencer uptime feed.

164

Design token vesting for team members and early investors.

Show answer

Use a vesting contract that holds allocated tokens and releases them on a schedule, typically a one-year cliff followed by monthly or linear vesting over two to four years. OpenZeppelin VestingWallet gives linear vesting per beneficiary; add a cliff and an owner-controlled revoke for employees who leave, returning unvested tokens. Make schedules public on-chain for transparency, and avoid admin functions that could release tokens early, as that erodes investor trust.

165

You are launching a 10,000-piece NFT collection and fear gas wars and bots. How do you design the mint?

Show answer

Avoid a first-come public rush. Use an allowlist phase with Merkle proofs or signatures, per-wallet limits, and staggered time slots. Consider a Dutch auction or raffle so price, not gas, decides allocation. Use ERC-721A or efficient batch minting to cut gas. Reveal traits later with Chainlink VRF so bots cannot snipe rare items. Load-test the front-end and RPC, and keep pause functions ready in case something goes wrong.

166

Users complain they received far fewer tokens than expected on large swaps. What is happening and how do you help?

Show answer

Large trades in shallow pools suffer price impact, and wide slippage settings let sandwich bots extract extra value. Show the expected output, price impact and minimum received before confirming. Set sensible default slippage, such as 0.5 percent for major pairs, and warn when impact is high. Use a router or aggregator that splits trades across pools, and offer private transaction submission so bots cannot see the swap in the public mempool.

167

A user lost funds on our DeFi app due to their own mistake and is angry. How would you handle it?

Show answer

Situation: a user sent tokens to the contract address directly instead of using deposit, and they were stuck. Task: support him honestly. Action: I listened, explained calmly what happened without blaming him, checked whether a recovery function existed, and escalated to the team. We had a rescue function for stray tokens behind the multisig, so after verification we returned them. Result: he thanked us publicly, and we added a front-end warning.

168

Tell me about working with product, legal or compliance teams on a blockchain feature.

Show answer

Situation: we planned a token reward feature for Indian users, and legal raised VDA tax and securities concerns. Task: as tech lead I had to find a compliant design. Action: I ran workshops to understand their constraints, proposed non-transferable points that convert to vouchers, and documented the data flow for compliance. Result: we launched a month later than planned but with sign-off from all teams, and avoided a costly redesign after launch.

169

Write the core reward-per-token logic for a staking rewards contract.

Show answer

uint256 public rewardRate; // tokens per second uint256 public rewardPerTokenStored; uint256 public lastUpdate; uint256 public totalStaked; mapping(address => uint256) public paid; mapping(address => uint256) public rewards; mapping(address => uint256) public staked; function rewardPerToken() public view returns (uint256) { if (totalStaked == 0) return rewardPerTokenStored; return rewardPerTokenStored + (block.timestamp - lastUpdate) * rewardRate * 1e18 / totalStaked; } modifier updateReward(address a) { rewardPerTokenStored = rewardPerToken(); lastUpdate = block.timestamp; rewards[a] += staked[a] * (rewardPerTokenStored - paid[a]) / 1e18; paid[a] = rewardPerTokenStored; _; }

170

How do you build a tokenised vault using ERC-4626?

Show answer

ERC-4626 standardises yield vaults: users deposit an asset and receive shares, and share price rises as the vault earns yield. import {ERC4626} from '@openzeppelin/contracts/token/ERC20/extensions/ERC4626.sol'; import {ERC20} from '@openzeppelin/contracts/token/ERC20/ERC20.sol'; import {IERC20} from '@openzeppelin/contracts/token/ERC20/IERC20.sol'; contract YieldVault is ERC4626 { constructor(IERC20 asset_) ERC20('Vault Share', 'vSHR') ERC4626(asset_) {} function totalAssets() public view override returns (uint256) { return IERC20(asset()).balanceOf(address(this)); // plus deployed funds } } OpenZeppelin uses virtual shares to defend against the first-depositor inflation attack.

171

Calculate impermanent loss when one token's price doubles.

Show answer

Formula: IL = 2 x sqrt(r) / (1 + r) - 1, where r is the price ratio change. For r = 2: sqrt(2) = 1.414, so IL = 2 x 1.414 / 3 - 1 = -0.057, about 5.7 percent. Example: deposit 1 ETH plus 3,000 USDC when ETH is 3,000 (6,000 dollars). ETH goes to 6,000. Holding would be 9,000 dollars; the pool position is worth about 8,485. Fees earned must exceed the 515 dollar gap for providing liquidity to beat holding.

172

A stablecoin used as collateral in your protocol loses its peg. How do you respond?

Show answer

Activate the risk plan: pause new borrowing against that asset and freeze its use as new collateral through governance or the guardian. Switch its oracle to a market price rather than a hard-coded one dollar, so positions are valued honestly, which may trigger liquidations. Reduce its loan-to-value and supply caps. Communicate clearly with users. Longer term, cap exposure per stablecoin, diversify collateral and model depeg scenarios in risk dashboards.

173

A sharp market fall causes cascading liquidations in your lending protocol and bad debt. How do you design against it?

Show answer

Set conservative loan-to-value ratios and liquidation thresholds per asset based on volatility and on-chain liquidity. Use supply and borrow caps for illiquid tokens, isolation mode for risky assets, and a liquidation bonus that attracts liquidators without over-penalising users. Allow partial liquidations so positions shrink gradually. Keep reliable oracles, an insurance or reserve fund to absorb bad debt, and run stress tests simulating 30 to 50 percent price drops.

174

A client in India wants to tokenise real estate for fractional ownership. What do you consider?

Show answer

Regulation comes first: fractional real estate is likely a security or collective investment, so SEBI rules, including those for fractional ownership platforms, apply, and crypto-like tokens are taxed as virtual digital assets at 30 percent with 1 percent TDS. Use a permissioned token with KYC-gated transfers, such as ERC-3643, and a legal SPV that holds the property. Keep title documents off-chain, put hashes on-chain, and plan for rent distribution, transfer restrictions and recovery.

175

A fee-on-transfer token breaks your DEX's accounting. What happened and how do you fix it?

Show answer

Fee-on-transfer tokens deduct a percentage on every transfer, so when your contract calls transferFrom for 100 tokens it may receive only 98. If the code assumes 100 arrived, balances and reserves drift and can be exploited. Fix it by measuring the actual amount received: read the contract's balance before and after the transfer and use the difference. Also decide whether to support such tokens at all and document unsupported token behaviours like rebasing.

176

An attacker borrows governance tokens with a flash loan to pass a malicious DAO proposal. How do you prevent it?

Show answer

Use snapshot-based voting power: count votes from token balances at a past block, as ERC20Votes does, so tokens acquired in the same transaction have no weight. Add a voting delay before voting starts, a quorum, and a timelock between approval and execution so the community can react or a guardian can veto. Require a proposal threshold, and keep critical actions such as treasury transfers behind additional safeguards like spending limits.

177

What does a typical dApp architecture look like?

Show answer

A dApp usually has smart contracts on a chain holding core logic and assets; a front-end, often React or Next.js, that connects to the user's wallet; a library such as ethers.js, viem or wagmi to read and write contracts through an RPC provider; an indexer or subgraph for fast queries of history; decentralised storage such as IPFS for files and metadata; and optional back-end services for notifications, relaying or off-chain computation, with oracles bringing external data on-chain.

178

What is IPFS and what is content addressing?

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IPFS, the InterPlanetary File System, is a peer-to-peer network for storing and sharing files. Instead of locating files by server address, it identifies them by a content identifier (CID) derived from the file's hash, which is content addressing. The same content always has the same CID, and any change gives a new CID, so data cannot be swapped silently. Files must be pinned by a node or pinning service, otherwise they may disappear.

179

How do you connect a browser wallet to a dApp using ethers.js?

Show answer

Browser wallets inject an EIP-1193 provider at window.ethereum. Wrap it with BrowserProvider and request accounts. import { BrowserProvider } from 'ethers'; async function connect() { if (!window.ethereum) throw new Error('Install a wallet'); const provider = new BrowserProvider(window.ethereum); const signer = await provider.getSigner(); // prompts the user const address = await signer.getAddress(); const { chainId } = await provider.getNetwork(); return { signer, address, chainId }; } In production, use EIP-6963 discovery or a connector library so users with several wallets can choose.

180

How do you call read and write functions of a contract with ethers v6?

Show answer

Create a Contract with address, ABI and a provider for reads or a signer for writes. import { Contract, parseEther } from 'ethers'; const abi = [ 'function count() view returns (uint256)', 'function increment() payable' ]; const readOnly = new Contract(ADDR, abi, provider); console.log(await readOnly.count()); // free eth_call const writable = new Contract(ADDR, abi, signer); const tx = await writable.increment({ value: parseEther('0.001') }); const receipt = await tx.wait(); console.log(receipt.hash, receipt.status);

181

What are the basic Foundry commands to create, build, test and work with a project?

Show answer

curl -L https://foundry.paradigm.xyz | bash && foundryup forge init my-dapp # new project with src, test, script forge install OpenZeppelin/openzeppelin-contracts forge build # compile forge test -vvv # run tests with traces forge coverage # coverage report forge snapshot # gas snapshot anvil # local node cast call ADDR 'count()(uint256)' --rpc-url $RPC Foundry tests are written in Solidity and run fast; Hardhat is the JavaScript or TypeScript alternative with a large plugin ecosystem.

182

How do you verify a deployed contract's source code on a block explorer?

Show answer

Verification uploads source and compiler settings so the explorer can recompile and match the bytecode, letting users read the code. # Foundry, using an explorer API key forge verify-contract 0xDeployedAddr src/Counter.sol:Counter \ --chain sepolia --etherscan-api-key $KEY --watch # With constructor arguments --constructor-args $(cast abi-encode 'constructor(address)' 0xOwner) # Hardhat npx hardhat verify --network sepolia 0xDeployedAddr 0xOwner Compiler version, optimiser runs and via-IR settings must match exactly.

183

Users report their USDC balance shows as a tiny fraction in your dApp. What is wrong?

Show answer

USDC uses 6 decimals, but the front-end probably formats every token with 18 decimals, so 100 USDC (100,000,000 base units) displays as 0.0000000001. Read decimals() from each token contract, or from a trusted token list, and use formatUnits and parseUnits with that value. Use bigint for amounts, never JavaScript floats, to avoid precision errors. Add tests with tokens of 6, 8 and 18 decimals.

184

Tell me about yourself for a blockchain developer role.

Show answer

Keep it to about a minute: present, past, future. For example: I am a computer science graduate who has spent the last year building on Ethereum. I have built three projects, including an NFT certificate system and a staking contract tested with Foundry fuzz tests, and deployed them on Sepolia with React front-ends. I practise security through Ethernaut challenges. I am looking for a role where I can write secure contracts and learn from experienced auditors.

185

Where do you see yourself in five years?

Show answer

I see myself as a senior smart contract engineer, owning the design and security of production protocols and mentoring juniors. In the next two years I want to master Solidity, testing and audit techniques, and contribute to open source. Later I would like to specialise in security or Layer 2 infrastructure. I hope to grow inside a team like this one, where the work is real and the standards are high.

186

How do you listen to contract events and query past events with ethers?

Show answer

Use contract.on for live events and queryFilter for history. const abi = ['event Transfer(address indexed from, address indexed to, uint256 value)']; const token = new Contract(TOKEN, abi, provider); token.on('Transfer', (from, to, value, event) => { console.log(from, to, value.toString(), event.log.blockNumber); }); const filter = token.filters.Transfer(null, MY_ADDRESS); const past = await token.queryFilter(filter, -5000); // last 5000 blocks console.log(past.length); Use a WebSocket provider for live events, and page large ranges because providers limit block spans.

187

How do you read a contract and send a transaction with viem?

Show answer

viem is a typed, lightweight alternative to ethers, and wagmi builds React hooks on it. import { createPublicClient, createWalletClient, http, custom } from 'viem'; import { sepolia } from 'viem/chains'; const pub = createPublicClient({ chain: sepolia, transport: http() }); const count = await pub.readContract({ address: ADDR, abi, functionName: 'count' }); const wallet = createWalletClient({ chain: sepolia, transport: custom(window.ethereum) }); const [account] = await wallet.requestAddresses(); const hash = await wallet.writeContract({ address: ADDR, abi, functionName: 'increment', account }); await pub.waitForTransactionReceipt({ hash });

188

Write a Foundry deployment script.

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// script/Deploy.s.sol pragma solidity ^0.8.24; import {Script, console} from 'forge-std/Script.sol'; import {Counter} from '../src/Counter.sol'; contract Deploy is Script { function run() external returns (Counter c) { vm.startBroadcast(); c = new Counter(); vm.stopBroadcast(); console.log('Counter deployed at', address(c)); } } # Run it with an encrypted keystore account forge script script/Deploy.s.sol --rpc-url $RPC_URL \ --account deployer --broadcast --verify Broadcast output in the broadcast folder records addresses for later scripts.

189

How do you store an NFT image on IPFS and use it in your contract?

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Upload the image to IPFS using a node or a pinning service, which returns a CID. Create a metadata JSON file whose image field is ipfs://<imageCID>, then upload the metadata folder to get a folder CID. Set the contract's base URI to ipfs://<folderCID>/ so tokenURI(5) resolves to the file for token 5. Pin the content with more than one provider, and use a gateway URL only for display in the front-end.

190

How do you test against real mainnet contracts using a fork?

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Anvil or Foundry tests can fork a live chain at a block, so you interact with real tokens and protocols locally. # Start a local fork anvil --fork-url $MAINNET_RPC --fork-block-number 21000000 # Or fork inside a test function setUp() public { vm.createSelectFork(vm.envString('MAINNET_RPC'), 21000000); } # Give yourself tokens or impersonate a whale deal(address(usdc), alice, 1_000e6); vm.prank(whale); Pinning the block number keeps results reproducible and makes RPC caching effective.

191

How do you handle a user on the wrong network in your dApp?

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Check chainId after connecting and listen for chainChanged. Then ask the wallet to switch, and add the chain if it is unknown. const target = '0xaa36a7'; // Sepolia 11155111 try { await window.ethereum.request({ method: 'wallet_switchEthereumChain', params: [{ chainId: target }] }); } catch (e) { if (e.code === 4902) { await window.ethereum.request({ method: 'wallet_addEthereumChain', params: [{ chainId: target, chainName: 'Sepolia', rpcUrls: [RPC], nativeCurrency: { name: 'ETH', symbol: 'ETH', decimals: 18 } }] }); } } Disable write buttons until the network is correct.

192

After a transaction is confirmed, your front-end still shows old data. How do you fix it?

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The UI is probably reading cached results or a lagging RPC node. After tx.wait() resolves, explicitly refetch or invalidate the relevant queries, for example with wagmi or TanStack Query invalidation. Show optimistic updates while pending. If you use an indexer, it may lag by a few blocks, so poll until it reaches the receipt's block number. Use the same RPC endpoint for writes and immediate reads, and avoid long caching for balances.

193

Your RPC provider starts rate-limiting and the dApp breaks during peak traffic. What do you do?

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Short term, add a fallback transport with multiple providers so requests fail over automatically, and use retries with backoff. Reduce load: batch calls with multicall, cache read results, use WebSockets instead of tight polling, and move history queries to an indexer. Long term, upgrade the plan, spread traffic across providers or run your own node for critical paths, and monitor error rates and latency with alerts so you learn of issues before users do.

194

Your fantasy sports contract needs live match scores. How do you get that data on-chain?

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Smart contracts cannot call web APIs, so use an oracle. Options include a decentralised oracle network such as Chainlink Functions to fetch from several sports APIs and aggregate, or an optimistic oracle where results are proposed and can be disputed. Settle only after the match result is final, use multiple data sources, add a dispute window for high-value pools, and handle cancelled matches. Avoid a single back-end key posting results, which is a central point of failure.

195

Your target users in India have never used crypto wallets. How do you design onboarding?

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Hide crypto complexity. Offer login with phone or email using embedded or smart contract wallets, so no seed phrase is needed at first. Sponsor gas with a paymaster so users need no tokens. Show amounts in rupees and plain language, support regional languages, and explain actions in simple terms. Provide account recovery via passkeys or guardians, and let advanced users export to self-custody later. Ensure KYC and tax requirements are clear where money is involved.

196

How do you handle unclear or changing requirements from a client?

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Situation: a client asked for a marketplace dApp but kept changing royalty and fee rules. Task: deliver without endless rework. Action: I wrote a short spec with examples and edge cases, got sign-off, and made fees configurable parameters with limits instead of hard-coded logic. Changes went through a simple change log with impact estimates. Result: we delivered in eight weeks, and later fee changes needed only a governance transaction, not a redeployment.

197

How would you implement Sign-In with Ethereum on a web back-end?

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Follow EIP-4361. The server generates a random nonce and stores it in the session. The front-end builds a SIWE message with domain, address, statement, URI, chain ID, nonce and issued-at time, and the wallet signs it. The server verifies the signature with a SIWE library, checks domain, nonce, expiry and chain, then issues a session cookie or JWT and deletes the nonce. For smart contract wallets, verify via ERC-1271 isValidSignature.

198

Design a back-end relayer so users can make transactions without holding gas.

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Users sign an EIP-712 meta-transaction or an ERC-4337 UserOperation. The relayer validates the signature, checks business rules and rate limits per user, then submits it, paying gas from a hot wallet or via a paymaster. Contracts must verify the original signer, using ERC-2771 trusted forwarders or account abstraction. Manage nonces carefully with a queue, replace stuck transactions, keep the relayer wallet funded and monitored, and cap daily sponsorship to prevent abuse.

199

How do you manage deployer private keys in a CI/CD pipeline?

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Never commit keys or put them in plain environment files. For testnets, use CI secret storage with restricted access. For mainnet, avoid hot keys in CI entirely: have CI produce a transaction proposal that signers approve in a multisig, or use a cloud KMS or HSM signer so the key never leaves secure hardware. Separate deployer and admin roles, transfer ownership to a multisig immediately after deployment, and log and review every deployment.

200

How do you monitor smart contracts in production?