Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Infosys hiring for Blockchain roles
Infosys is an Indian multinational IT services and consulting company offering digital transformation, cloud, application management, testing and data services to global clients. It is known for its Mysuru training campus where new hires go through foundation programmes. In India it recruits for roles such as systems engineer, test engineer, data analyst and data engineer, often through campus and off-campus drives.
For Blockchain roles, this Infosys practice set focuses most on Consensus and Scaling (6), Solidity Programming (6), Blockchain Fundamentals (5), Smart Contract Security (4). It has 12 easy, 18 medium questions, so you can start easy and work up to the harder ones.
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
Why should you check return values of low-level calls and use SafeERC20?
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).
02
How do you compute a keccak256 hash in Solidity and in ethers.js?
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.
03
What is a hash collision and why does it matter for blockchains?
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 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.
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.
06
What are Delegated Proof of Stake and Proof of Authority?
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.
07
How is a blockchain different from a traditional database?
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.
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.
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.
10
Compare interfaces, abstract contracts and libraries.
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.
11
How would you set up a local blockchain for development and testing?
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.
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.
13
Write a Foundry unit test and a fuzz test for a contract.
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.
14
How can a smart contract suffer denial of service?
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.
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.
16
How do you approve and transfer ERC-20 tokens from a script with ethers.js?
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.
18
Write a Python function that computes a Merkle root from a list of transactions.
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']))
19
What are state channels, such as the Lightning Network?
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.
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.
21
Write a simple Proof of Work loop in Python with a difficulty setting.
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.
22
How did integer overflow bugs happen before Solidity 0.8?
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.
23
How are mappings stored, and why can you not iterate over them?
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.
24
How do you get current fee data and send an EIP-1559 transaction with ethers.js?
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.
26
Explain public, external, internal and private visibility.
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.
27
How do you calculate a borrower's health factor in a lending protocol?
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.
28
What is the difference between a soft fork and a hard fork?
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.
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.
30
How do you get the latest block number from an Ethereum node using JSON-RPC?
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.
Tips to crack the Infosys Blockchain interview
Practise pseudocode output questions and puzzle-style reasoning, which commonly appear in Infosys online tests and HackWithInfy style contests.
Revise SQL joins, normalisation and one programming language well enough to write and explain simple programs on paper.
Mention willingness to learn new technologies and relocate, since Infosys emphasises its training programme and flexible project allocation.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; Infosys interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Infosys Blockchain interview?
Candidates usually go through 4 stages: Online assessment (aptitude, reasoning + pseudocode); Coding test; Technical interview; HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the Infosys Blockchain interview?
This practice set leans on Consensus and Scaling (6), Solidity Programming (6), Blockchain Fundamentals (5), Smart Contract Security (4). Revise those first, then practise explaining one or two of your own projects end to end, because most Infosys interviewers ask you to walk through real work.
Are these the exact questions asked at Infosys?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like Infosys's. They are not a leaked or verified list of past questions.
Where does Infosys hire Blockchain professionals in India?
Infosys has large teams in Bengaluru, Mysuru, Pune, Hyderabad, Chennai. Openings and locations change often, so check the Infosys careers page and filter by city and experience.