Blockchain and its Applications Week 1 NPTEL Assignment Answers 2026

1. Which of the following statements is true regarding the foundational concepts of blockchain and
cryptography?

a) Decentralization in blockchain ensures that a single authority controls the network for higher efficiency.
b) SHA-256 is a cryptographic hash function widely used in blockchain due to its fixed output size and collision resistance.
c) A hash chain is a sequence of cryptographic keys used to decode blockchain data.
d) Cryptographic hash ensures that the blockchain data cannot be read by anyone outside the network.

Answer : See Answers

2. onsider the following input string: “Hello World”. You apply SHA-256 to this string. Then, you chang
le input to “Hello World!” (added an exclamation mark) and apply SHA-256 again. Which statemer
describes the relationship between the two output hashes?

a) The two hashes will be identical.
b) The second hash will be the same as the first, plus a few extra bits at the end
c) The two hashes will be completely different and uncorrelated.
d) The second hash will be exactly 8 bits longer than the first.

Answer :

3. A blockchain network uses SHA-256 for its hashing process. If it takes 10-6 seconds to compute ingle SHA-256 hash, how long would it take (approximately) to compute 2128 hashes for a collisio attack?

a) 10¹⁰ years
b) 10¹⁵ years
c) 10²⁰ years
d) 10²⁵ years

Answer :

4. In a decentralized distributed system with 100 participants, which of the following statements is true
regarding trust and communication?

a) At least 50 participants must trust each other for the system to function.
b) A central body governing communication among all 100 participants is mandatory.
c) Participants may or may not trust each other, as the system ensures integrity using cryptographic protocols and agreement through consensus protocols.
d) All the 100 participants must trust each other.

Answer :

5. A blockchain network achieves an average block generation time of 5 minutes under normal conditions. However, due to scheduled maintenance, the network’s hash rate is reduced by 50% for 4 hours daily. If the network operates for 12 hours in total (including the maintenance period), how many blocks will be added to the blockchain?

a) 120
b) 200
c) 216
d) 240

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6. Where are the transaction logs stored in a blockchain network?

a) In a centralized SQL database.
b) On an immutable ledger controlled by a central authority.
c) In metadata tables on each peer.
d) In the distributed ledger of each peer across the network.

Answer :

7. An attacker wants to find a collision in a cryptographic hash function with a 128-bit output. What is
the approximate number of hash operations required to succeed?

a) 2⁶⁴
b) 2²⁵⁶
c) 2¹²⁸
d) 0.5 × 2¹²⁸

Answer :

8. Which of the following statements accurately describes a blockchain?

a) A centralized database where data is stored on a single server.
b) A distributed ledger where data is stored across multiple nodes and is immutable.
c) A system that only stores cryptocurrency transaction data on a single node.
d) A network that uses a single user to control access and updates to the data.

Answer :

9. A centralized database processes transactions at 10,000 TPS (Transactions Per Second). You decide to move this system to a decentralized blockchain to increase trust. Which of the following is the most likely outcome regarding performance and control?

a) Throughput increases, and a single authority retains control.
b) Throughput decreases, and control is distributed among peers.
c) Throughput remains the same, but data becomes mutable.
d) Throughput increases, and data becomes encrypted

Answer :

10. Which of the following describes the avalanche effect in a cryptographic hash function?

a) Given the same input, the hash function returns a different hash 99.99% of the time.
b) It takes 10% attempts to reverse-engineer the original message from the hash.
c) A small change in the input causes a drastic change in the hash, flipping nearly all the bits.
d) The hash function always returns the same hash for the same input.

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NPTEL Blockchain and its Applications Week 1 Assignment Answers 2024

Q1. What are the features of a hash function?

a. Puzzle-friendly
b. Collision-resistance
c. Deterministic
d. Post image resistance

Answer: a, b, c
Explanation:

• Puzzle-friendly: The hash must be hard to reverse-engineer; useful in mining (Proof of Work).
• Collision-resistance: It should be extremely unlikely that two different inputs produce the same hash.
• Deterministic: The same input always results in the same hash output.
• Post image resistance is not a standard term in cryptographic hash functions.

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Q4. Which of the statements below is/are true for decentralized distributed systems?

a. Players may or may not trust each other
b. Players must trust each other
c. Central body should govern the communication
d. None of the above

Answer: a
Explanation:
In decentralized systems like blockchain, trust is minimized between participants. They may not trust each other, but consensus protocols maintain system integrity.

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Q5. Miner nodes only execute new transactions but cannot verify previous transaction hash?

a. True
b. False

Answer: b
Explanation:
False – Miner nodes not only execute new transactions but also verify the validity of previous transaction hashes to ensure integrity.

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Q6. Which of the following is/are true for blockchains?

a. Works based on Push technique
b. Existing data can be deleted easily
c. Tamper-proof
d. None of the above

Answer: a, c
Explanation:

• Push technique: Data is broadcasted across the network (push, not pull).
• Tamper-proof: Due to cryptographic linking, altering data is computationally expensive.
• Data can’t be deleted easily, making option b incorrect.

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Q7. Where are the ledger logs stored in a blockchain?

a. On a SQL Database
b. On a central immutable ledger
c. On a metadata table
d. In ledger of each peer

Answer: d
Explanation:
Each node (peer) maintains a copy of the ledger, making the blockchain decentralized and fault-tolerant.

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Q8. Which of the following is an avalanche effect to a cryptographic hash function?

a. Given the same message the hash function would not return the same hash
b. It is not very difficult to generate the original message from the hash
c. A small change in the message impacts large change in the hash value
d. None of the above

Answer: c
Explanation:
The avalanche effect ensures that a tiny change in input causes major changes in output hash, which improves security.

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Q9. Genesis blocks may not contain the

a. First transaction
b. First transaction block
c. Last transaction block
d. None of the above

Answer: c
Explanation:
The genesis block is the first block in a blockchain. It can’t contain the last transaction block.

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Q10. Which of the below is/are blockchain-based app examples?

a. Cross-border payments
b. Supply chain
c. Anti-money laundering tracking system
d. UTXO

Answer: a, b, c
Explanation:

• Blockchain is widely used in cross-border payments, supply chain tracking, and AML systems for transparency.
• UTXO (Unspent Transaction Output) is a mechanism, not an application.