NPTEL Blockchain and its Applications Week 1 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.
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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
с) 1 × 2256
d) 0.5 × 2256
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3. A blockchain network uses SHA-256 for its hashing process. If it takes 10-6 seconds to compute a single SHA-256 hash, how long would it take (approximately) to compute 2128 hashes for a collision 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.
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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.
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7. 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 105 attempts to reverse-engineer the original message from the hash.
a) A small change in the input causes a drastic change in the hash, flipping nearly all the bits.
c) The hash function always returns the same hash for the same input.
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8. Which of the following statements accurately describes a blockchain?
a) A centralized database where data is stored on a single server.
b) distributed ledger where data is stored across multiple nodes and is immutable.
c) 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.
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9. Which of the following is/are possible use cases of blockchain technology?
a) Cross-border payments
b) Supply chain management
c) Centralized Anti-money laundering tracking system
d) Maintaining data over a single database server
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10. In a blockchain using SHA-256, if the hashes of strings A and B are concatenated and then hashed again, what is the length of the final hash?
a) 256 bits
b) 512 bits
c) 128 bits
d) 1024 bits
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