CBSE Consensus Algorithm Security 2 — Questions and Answers
Question 1: In a Proof-of-Work blockchain, what condition must an attacker satisfy to execute a 51% attack successfully?
- Control more than half of the network's total hash rate (Correct answer)
- Own more than half of all circulating tokens
- Compromise more than half of all validator nodes
- Possess private keys for more than half of all wallets
Correct answer: Control more than half of the network's total hash rate
A 51% attack requires the attacker to control the majority of the network's computational hash rate, enabling them to rewrite recent blocks and double-spend.
Question 2: Which consensus mechanism is most vulnerable to a 'long-range attack' where an adversary rewrites blockchain history from genesis?
- Proof of Work
- Proof of Stake (Correct answer)
- Delegated Proof of Stake
- Proof of Authority
Correct answer: Proof of Stake
Pure Proof-of-Stake systems are susceptible to long-range attacks because old private keys can be used to rewrite history without requiring ongoing computational resources.
Question 3: What is a 'nothing-at-stake' problem in Proof-of-Stake consensus?
- Validators can vote on multiple competing forks at no additional cost (Correct answer)
- Validators lose all staked funds if they go offline
- Nodes have no incentive to participate in block validation
- Attackers can stake zero tokens to gain consensus power
Correct answer: Validators can vote on multiple competing forks at no additional cost
Nothing-at-stake means validators can simultaneously vote on all fork candidates without penalty, undermining finality and enabling double-spend attacks.
Question 4: Slashing conditions in Ethereum's Casper PoS protocol are designed primarily to penalize which behavior?
- Equivocation — signing two conflicting blocks at the same height (Correct answer)
- Failing to produce a block when selected as proposer
- Staking below the minimum required validator balance
- Connecting to too many peer nodes simultaneously
Correct answer: Equivocation — signing two conflicting blocks at the same height
Slashing penalizes equivocation (double-voting or double-proposing) by destroying a portion of the offending validator's staked ETH.
Question 5: In Byzantine Fault Tolerant (BFT) consensus, what is the maximum fraction of faulty nodes the system can tolerate while still reaching agreement?
- Less than one-third of all nodes (Correct answer)
- Less than one-half of all nodes
- Less than two-thirds of all nodes
- Any fraction, as long as faulty nodes are identified
Correct answer: Less than one-third of all nodes
Classical BFT requires that fewer than one-third of nodes are Byzantine (malicious or faulty) to guarantee safety and liveness.
Question 6: What attack involves an adversary selectively withholding a found block to gain a statistical mining advantage in PoW?
- Eclipse attack
- Selfish mining attack (Correct answer)
- Sybil attack
- Grinding attack
Correct answer: Selfish mining attack
In selfish mining, a miner secretly extends their private chain and releases blocks strategically to waste honest miners' work, gaining a disproportionate share of rewards.
Question 7: Which mitigation technique directly addresses the nothing-at-stake problem in Proof-of-Stake systems?
- Increasing block size to reduce fork frequency
- Implementing slashing penalties for validators who sign conflicting forks (Correct answer)
- Requiring validators to wait 30 days before unstaking
- Using hash-based puzzles to validate stake commitments
Correct answer: Implementing slashing penalties for validators who sign conflicting forks
Slashing creates an economic cost for signing multiple forks, making it irrational for validators to support competing chains simultaneously.
CBSE Consensus Algorithm Security 2
In a Proof-of-Work blockchain, what condition must an attacker satisfy to execute a 51% attack successfully?