NPTEL Blockchain and its Applications Week 1 And 2 Assignment Answers 2025

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: b
Explanation: SHA-256 is crucial in blockchain for its fixed 256-bit output, collision resistance, and determinism.

2. An attacker wants to find a collision in a cryptographic hash function with a 256-bit output. What is the approximate number of hash operations required to succeed?

a) 1 × 2¹²⁸ ✅
b) 0.75 × 2¹²⁸
c) 1 × 2²⁵⁶
d) 0.5 × 2²⁵⁶

✅ Answer: a
Explanation: Due to the Birthday Paradox, a collision in a 256-bit hash occurs in approximately 2¹²⁸ operations.

3. If it takes 10⁻⁶ seconds to compute one SHA-256 hash, how long to compute 2¹²⁸ hashes?

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

✅ Answer: d
Explanation: 2¹²⁸ ≈ 3.4 × 10³⁸ hashes. At 10⁶ hashes/sec, it takes ~3.4 × 10³² seconds ≈ 10²⁵ years.

4. In a decentralized distributed system with 100 participants, which statement is true?

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

✅ Answer: c
Explanation: Blockchain ensures trust via cryptography and consensus protocols, not individual trust.

5. Block generation every 5 minutes. Hashrate halved for 4 out of 12 hours. How many blocks added?

a) 120 ✅
b) 200
c) 216
d) 240

✅ Answer: a
Explanation:

Full speed: 8 hours → 8×60 / 5 = 96 blocks
Half speed: 4 hours → 4×60 / 10 = 24 blocks
Total = 96 + 24 = 120 blocks

6. Where are transaction logs stored in a blockchain?

a) Centralized SQL database
b) Immutable ledger controlled by a central authority
c) Metadata tables on each peer
d) Distributed ledger of each peer across the network ✅

✅ Answer: d
Explanation: Blockchain is distributed, and each peer/node maintains a copy of the ledger.

7. What is the Avalanche Effect in cryptographic hash functions?

a) Same input gives different hash 99.99% of the time
b) Takes 10⁵ attempts to reverse hash
c) A small change in input flips most of the output bits ✅
d) Hash function gives same output for same input

✅ Answer: c
Explanation: The avalanche effect ensures that even one bit change in input causes large change in hash.

8. Which of the following describes a blockchain?

a) Centralized database
b) Distributed ledger stored across multiple nodes and is immutable ✅
c) Stores crypto transactions on a single node
d) Controlled by a single user

✅ Answer: b
Explanation: Blockchain is a decentralized, distributed, and immutable ledger system.

9. Which are valid blockchain use cases?

a) Cross-border payments ✅
b) Supply chain management ✅
c) Centralized AML tracking
d) Data on single DB server

✅ Answer: a, b
Explanation: Blockchain supports decentralized tracking, making it ideal for finance and supply chain.

10. SHA-256 hash of concatenated hashes of strings A and B — what is the final hash length?

a) 256 bits ✅
b) 512 bits
c) 128 bits
d) 1024 bits

✅ Answer: a
Explanation: No matter input size, SHA-256 always outputs 256 bits.

NPTEL Blockchain and its Applications Week 2 Assignment Answers

1. RSA private key for p=11, q=19, e=11. What is d?

a) 35
b) 131 ✅
c) 101
d) 149

✅ Answer: b
Explanation:

n=p×q=11×19=209n = p \times q = 11 \times 19 = 209n=p×q=11×19=209
ϕ(n)=(11−1)(19−1)=10×18=180\phi(n) = (11 – 1)(19 – 1) = 10 \times 18 = 180ϕ(n)=(11−1)(19−1)=10×18=180
e=11e = 11e=11, we find d such that (d × 11) mod 180 = 1
Using Extended Euclidean Algorithm, d = 131

2. Steps for sending message with confidentiality and integrity?

a) public, hash, private, public, private
b) private, message, public, private, public
c) public, hash, private, private, public ✅
d) public, hash, private, public, public

✅ Answer: c
Explanation:

Encrypt with Bob’s public key (confidentiality)
Sign hash with Alice’s private key (integrity & non-repudiation)
Bob decrypts with his private key, then verifies signature with Alice’s public key

3. What does digital signature in blockchain ensure?

a) Sender can deny
b) Ensures sender cannot deny, and receiver can verify ✅
c) Only provides encryption
d) Only ensures confidentiality

✅ Answer: b
Explanation:
Digital signatures ensure non-repudiation and verifiability of the sender.

4. Why does Alice sign with her private key?

a) Encrypt the message
b) Prevent others from reading
c) Prove the message came from Alice ✅
d) Hide contents

✅ Answer: c
Explanation:
Signing proves authenticity—only Alice has access to her private key.

5. Correct post-order traversal of the Merkle Tree?

a) {…}
b) {…}
c) {1, 2, 12, 3, 4, 34, 1234, 5, 6, 56, 56, 5656, 12345656} ✅
d) {…}

✅ Answer: c
Explanation:
Post-order: left subtree → right subtree → parent (applies recursively to entire tree)

6. What refers to a block in a blockchain?

a) Future nonce
b) Block size
c) Previous Block Hash ✅
d) Timestamp

✅ Answer: c
Explanation:
Each block has a hash of the previous block, ensuring immutability and integrity.

7. What is NOT aligned with cryptocurrency goals?

a) Decentralization
b) Immutability
c) Centralized control ✅
d) Transparency

✅ Answer: c
Explanation:
Cryptocurrencies are built to remove central control—this violates core principles.

8. Which statements about Bitcoin are true?

1, 2, 3 ✅
2, 3, 4
1, 2, 4
1, 3, 4

✅ Answer: a
Explanation:
Bitcoin:

Uses Proof of Work
Has P2P architecture
Miners get rewards
(Statement 4 is false, Bitcoin does not use PoS)

9. What is ‘safety’ in Bitcoin?

a) Preventing invalid transactions ✅
b) Speed
c) Transaction privacy
d) Max TPS

✅ Answer: a
Explanation:
Safety ensures only valid blocks and transactions are added to the chain.

10. What is the goal of consensus in distributed systems?

a) Same speed
b) Agreement on single value/state ✅
c) Fewer nodes
d) Encrypt all data

✅ Answer: b
Explanation:
Consensus protocols (e.g., PoW, PoS) ensure all honest nodes agree on the same blockchain state.