Blockchain Security Training Consensus Mechanism Vulnerabilities 2 — Questions and Answers
Question 1: In a 51% attack on a Proof-of-Work blockchain, what can the attacker primarily accomplish?
- Steal private keys from any wallet
- Double-spend coins by reorganizing the chain (Correct answer)
- Mint unlimited new tokens at will
- Read encrypted transaction contents
Correct answer: Double-spend coins by reorganizing the chain
Controlling a majority of hash power lets an attacker rewrite recent blocks and double-spend, but not forge signatures or mint arbitrary coins.
Question 2: Which type of network is MOST economically vulnerable to a 51% attack?
- Bitcoin mainnet
- A small-cap PoW altcoin with low total hash rate (Correct answer)
- Ethereum after the Merge
- A permissioned enterprise chain
Correct answer: A small-cap PoW altcoin with low total hash rate
Low total hash rate means renting enough hashing power to overpower the network is cheap, making small PoW coins prime targets.
Question 3: What is 'selfish mining'?
- Mining only your own transactions
- Withholding found blocks to gain a disproportionate share of rewards (Correct answer)
- Refusing to share mining pool profits
- Mining without paying electricity costs
Correct answer: Withholding found blocks to gain a disproportionate share of rewards
Selfish miners withhold newly found blocks and release them strategically to waste honest miners' work and capture more than their fair reward share.
Question 4: A 'finality reversion' attack is most concerning because it undermines which property?
- Transaction privacy
- The guarantee that confirmed transactions are irreversible (Correct answer)
- Smart contract gas pricing
- Wallet address generation
Correct answer: The guarantee that confirmed transactions are irreversible
Finality means confirmed transactions cannot be undone; reverting it breaks settlement assurance that users and exchanges rely on.
Question 5: Why does increasing the number of block confirmations reduce double-spend risk?
- It encrypts the transaction more strongly
- Each added block makes reorganizing the chain exponentially harder (Correct answer)
- It lowers transaction fees
- It hides the sender's identity
Correct answer: Each added block makes reorganizing the chain exponentially harder
More confirmations bury a transaction under additional proof-of-work, so an attacker must out-mine an ever-growing portion of the chain to reverse it.
Question 6: In Proof-of-Stake, a 'long-range attack' typically exploits what?
- High electricity consumption
- Old validator keys whose stake has since been withdrawn (Correct answer)
- Faster block propagation
- Quantum computing weaknesses
Correct answer: Old validator keys whose stake has since been withdrawn
Long-range attacks use previously-bonded keys (whose stake is no longer at risk) to rewrite history from an old point, defended against with checkpoints/weak subjectivity.
Question 7: Which mitigation makes a 51% attack on PoW more expensive to execute?
- Reducing block size
- A larger, more decentralized pool of honest hash power (Correct answer)
- Disabling transaction fees
- Shorter confirmation times
Correct answer: A larger, more decentralized pool of honest hash power
The more honest hash power securing the chain, the more expensive it becomes for an attacker to acquire a competing majority.
Blockchain Security Training Consensus Mechanism Vulnerabilities 2
In a 51% attack on a Proof-of-Work blockchain, what can the attacker primarily accomplish?