Cryptocurrency Blockchain and Consensus Mechanisms 2 — Questions and Answers
Question 1: What is a 51% attack in a proof-of-work blockchain?
- A bug that crashes 51% of nodes
- When a single entity controls more than half the network's mining hash rate (Correct answer)
- A smart contract vulnerability affecting 51% of transactions
- A denial-of-service attack targeting 51 specific validators
Correct answer: When a single entity controls more than half the network's mining hash rate
A 51% attack occurs when one miner or pool controls the majority of hash rate, allowing them to rewrite recent transaction history.
Question 2: What is the purpose of a Merkle tree in a blockchain block?
- To encrypt private keys for wallet storage
- To efficiently summarize and verify all transactions in a block (Correct answer)
- To link blocks together via hash pointers
- To store the coinbase reward address
Correct answer: To efficiently summarize and verify all transactions in a block
A Merkle tree hashes transaction pairs recursively into a single Merkle root, enabling efficient and tamper-evident verification of any transaction.
Question 3: In Ethereum's proof-of-stake, what happens to a validator that behaves dishonestly?
- Their node is blacklisted by DNS
- Their staked ETH is partially or fully slashed (Correct answer)
- They are redirected to a testnet automatically
- Their rewards are delayed by 7 epochs
Correct answer: Their staked ETH is partially or fully slashed
Slashing penalizes dishonest validators by destroying a portion of their staked ETH, making attacks financially costly.
Question 4: Which consensus property ensures that all honest nodes eventually agree on the same chain?
- Liveness
- Finality
- Safety
- Agreement (Correct answer)
Correct answer: Agreement
Agreement (also called consistency) guarantees that all honest nodes converge on the same blockchain state.
Question 5: What distinguishes a hard fork from a soft fork?
- Hard forks only affect miners; soft forks affect all nodes
- Hard forks are backward-incompatible, splitting the chain if not universally adopted (Correct answer)
- Soft forks require more than 90% miner approval while hard forks require 51%
- Hard forks change block size only; soft forks change transaction format only
Correct answer: Hard forks are backward-incompatible, splitting the chain if not universally adopted
A hard fork introduces rules that old nodes reject, potentially creating two separate chains if adoption is not unanimous.
Question 6: What is the primary role of the nonce in Bitcoin's proof-of-work?
- It uniquely identifies each transaction in the mempool
- It is incremented by miners to find a hash below the target difficulty (Correct answer)
- It prevents double-spending by tracking UTXO state
- It encrypts the coinbase transaction for the miner
Correct answer: It is incremented by miners to find a hash below the target difficulty
Miners repeatedly change the nonce and rehash the block header until the resulting hash meets the current difficulty target.
Question 7: What problem does the Byzantine Generals Problem model in distributed blockchain networks?
- How to compress large blocks across slow network links
- How nodes can reach consensus when some participants may be malicious or faulty (Correct answer)
- How to select a leader node for block production fairly
- How to synchronize clocks across geographically distributed nodes
Correct answer: How nodes can reach consensus when some participants may be malicious or faulty
The Byzantine Generals Problem describes the challenge of achieving reliable consensus when some participants may send conflicting or dishonest messages.
Cryptocurrency Blockchain and Consensus Mechanisms 2
What is a 51% attack in a proof-of-work blockchain?