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Blockchain Telephonic Interview Questions & Answers

30 questions for the telephonic interview stage — 19 easy, 11 medium.

About the telephonic interview round

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Blockchain concepts clearly without a screen to point at.

Tips for the telephonic interview round

  • Keep your CV in front of you and be ready to explain every line on it.
  • Answer in short, clear sentences; there is no whiteboard, so structure matters.
  • Take the call somewhere quiet and confirm the time in advance.
  • Prepare one question to ask the interviewer at the end.
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01

What are common access control vulnerabilities?

Smart Contract SecurityMedium
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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.
02

What is a Layer 2 and why is it needed?

Consensus and ScalingEasy
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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.
03

How can a smart contract suffer denial of service?

Smart Contract SecurityMedium
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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.
04

Why should you not use tx.origin for authorisation?

Smart Contract SecurityEasy
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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.
05

What is IPFS and what is content addressing?

dApp DevelopmentEasy
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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.
06

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

Consensus and ScalingMedium
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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.
07

What was the Merge?

Ethereum and the EVMEasy
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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.
08

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

Cryptography for BlockchainEasy
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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.
09

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

Ethereum and the EVMMedium
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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.
10

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

Cryptography for BlockchainEasy
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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.
11

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

Cryptography for BlockchainEasy
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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.
12

What types of stablecoins exist?

DeFi, Tokens and NFTsEasy
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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.
13

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

Blockchain FundamentalsMedium
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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.
14

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

Blockchain FundamentalsMedium
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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.
15

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

Blockchain FundamentalsEasy
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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.
16

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

DeFi, Tokens and NFTsMedium
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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.
17

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

DeFi, Tokens and NFTsEasy
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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.
18

What is DeFi?

DeFi, Tokens and NFTsEasy
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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.
19

Explain gas, gas limit and gas price.

Ethereum and the EVMEasy
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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.
20

Tell me about yourself for a blockchain developer role.

dApp DevelopmentEasy
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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.
21

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

Cryptography for BlockchainEasy
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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.
22

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

Solidity ProgrammingEasy
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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.
23

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

dApp DevelopmentEasy
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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.
24

What is a reentrancy attack?

Smart Contract SecurityMedium
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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.
25

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

Smart Contract SecurityMedium
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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.
26

How is an Ethereum address derived from a private key?

Cryptography for BlockchainMedium
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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.
27

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

Ethereum and the EVMMedium
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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.
28

What is Solidity?

Solidity ProgrammingEasy
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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.
29

How does a blockchain achieve immutability?

Blockchain FundamentalsEasy
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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.
30

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

Ethereum and the EVMEasy
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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.

Frequently asked questions

What happens in the Blockchain telephonic interview round?

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Blockchain concepts clearly without a screen to point at.

How do I clear the telephonic interview round?

Keep your CV in front of you and be ready to explain every line on it. Answer in short, clear sentences; there is no whiteboard, so structure matters. Take the call somewhere quiet and confirm the time in advance.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

Online Assessment questionsTechnical Round questionsManagerial Round questionsRapid-Fire questionsHR & behavioural questionsFull mock interview

More ways to practise

Company-wise Blockchain questionsExperience-wise Blockchain questionsRole-wise Blockchain questionsCity-wise Blockchain questionsMost asked Blockchain questionsScenario-based questionsPractical / hands-on questionsHR & behavioural questionsBlockchain mock interviewOnline assessment questionsTechnical round questionsManagerial round questionsBlockchain topic sheets