Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About TCS hiring for Blockchain roles
Tata Consultancy Services is a Tata Group company and one of the largest IT services and consulting firms in the world. It delivers application development, testing, cloud, data and analytics services to clients in banking, retail, manufacturing and other industries. In India it hires freshers and experienced staff for software testing, data engineering, BI reporting and analytics roles across its delivery centres.
For Blockchain roles, this TCS practice set focuses most on Consensus and Scaling (7), Cryptography for Blockchain (7), Solidity Programming (3), Smart Contract Security (3). It has 12 easy, 18 medium questions, so you can start easy and work up to the harder ones.
Company type
IT services
Headquarters
Mumbai, India
Main India offices
Mumbai, Chennai, Pune, Bengaluru, Hyderabad
TCS Blockchain interview process
TCS NQT online assessment (aptitude + coding)
Technical interview
Managerial round
HR round
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
How do you read the latest finalized block with ethers.js, and why use it?
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.
02
How would you check on-chain whether a user is in a 10,000-address allowlist without storing all addresses?
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.
03
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.
04
What does a smart contract audit process look like?
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.
05
What is Bitcoin halving and difficulty adjustment?
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.
06
Calculate the output of a swap in a constant product pool with a 0.3 percent fee.
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.
07
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.
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.
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.
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.
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.
12
What is the difference between an execution client and a consensus client, and why does client diversity matter?
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.
13
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']))
14
What is the difference between an externally owned account and a contract account?
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.
15
What does a block contain and what is inside a block header?
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.
16
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.
17
What is the difference between optimistic rollups and zk-rollups?
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.
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.
19
A developer accidentally pushed a private key to a public GitHub repo. What do you do?
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.
20
What are seed phrases and HD wallets (BIP-39, BIP-32, BIP-44)?
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.
21
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.
22
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).
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.
24
How is public-key cryptography used in blockchain?
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.
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.
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.
27
Write a basic voting contract using a struct, mapping and event.
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.
Tips to crack the TCS Blockchain interview
Practise TCS NQT style numerical, verbal and reasoning sections along with two basic coding problems under strict time limits.
Be ready to explain your final-year project, SQL basics and core concepts like OOP, DBMS and software testing life cycle.
Read about TCS service lines such as BFSI and its iON and Ignio platforms to answer why you want to join.
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; TCS interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the TCS Blockchain interview?
Candidates usually go through 4 stages: TCS NQT online assessment (aptitude + coding); Technical interview; Managerial round; 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 TCS Blockchain interview?
This practice set leans on Consensus and Scaling (7), Cryptography for Blockchain (7), Solidity Programming (3), Smart Contract Security (3). Revise those first, then practise explaining one or two of your own projects end to end, because most TCS interviewers ask you to walk through real work.
Are these the exact questions asked at TCS?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like TCS's. They are not a leaked or verified list of past questions.
Where does TCS hire Blockchain professionals in India?
TCS has large teams in Mumbai, Chennai, Pune, Bengaluru, Hyderabad. Openings and locations change often, so check the TCS careers page and filter by city and experience.