CBSE - Certified Blockchain Security Expert Consensus Algorithm Security Questions and Answers 1 — Questions and Answers
Question 1: A Proof-of-Stake blockchain is experiencing a chain fork. A validator decides to validate transactions and create blocks on both forked chains simultaneously to maximize their potential rewards, regardless of which chain ultimately becomes canonical. What specific security vulnerability does this action represent?
- Selfish Mining
- 51% Attack
- Nothing-at-Stake Problem (Correct answer)
- Eclipse Attack
Correct answer: Nothing-at-Stake Problem
The Nothing-at-Stake problem is unique to Proof-of-Stake consensus mechanisms. It describes a scenario where, because creating blocks has a low marginal cost, validators are incentivized to vote for multiple blockchain forks, as there is no financial penalty for doing so. This behavior can destabilize the consensus process and make the network more susceptible to double-spending attacks.
Question 2: In a Proof-of-Work based cryptocurrency network, a mining pool has just acquired 52% of the total network hash rate. Which of the following malicious activities can this pool now successfully execute?
- Steal private keys from user wallets.
- Change the fundamental consensus rules of the protocol.
- Reverse its own transactions that have already been confirmed. (Correct answer)
- Create new coins out of thin air, violating the supply limit.
Correct answer: Reverse its own transactions that have already been confirmed.
A 51% attack grants the attacker control over the block creation process. This allows them to create a private fork of the blockchain, spend coins on the original chain, and then release their longer private chain to the network, effectively reversing the original transaction (double-spending). They cannot, however, steal private keys, change protocol rules without a hard fork, or create coins outside of the established block reward rules.
Question 3: A private consortium blockchain uses the Practical Byzantine Fault Tolerance (PBFT) consensus algorithm. The network consists of 10 validator nodes. What is the maximum number of malicious or faulty nodes the network can withstand while still guaranteeing consensus?
- 1
- 5
- 4
- 3 (Correct answer)
Correct answer: 3
Practical Byzantine Fault Tolerance (PBFT) is designed to function correctly as long as the number of faulty or malicious nodes (f) is less than one-third of the total number of nodes (n). The formula is (n-1)/3. In a network with 10 nodes, n=10. The maximum number of faulty nodes (f) would be (10-1)/3 = 3.
Question 4: Which of the following consensus algorithms is most vulnerable to a Sybil attack where an attacker creates a large number of pseudonymous identities to gain disproportionate influence, assuming no significant economic barrier is in place?
- Proof of Work (PoW)
- Proof of Stake (PoS)
- A basic voting-based consensus without economic staking (Correct answer)
- Practical Byzantine Fault Tolerance (PBFT) in a permissioned network
Correct answer: A basic voting-based consensus without economic staking
A Sybil attack involves creating numerous fake identities to undermine a system. In a basic voting system where each identity gets one vote, an attacker can easily create many identities to control the outcome. PoW and PoS are specifically designed to be Sybil-resistant by requiring a costly resource (computational power or staked capital) to participate, making it prohibitively expensive to create a controlling number of 'identities'. PBFT in a permissioned network relies on a known, fixed set of validators, mitigating this risk.
Question 5: A security analyst is reviewing a new public blockchain protocol. The protocol uses a Proof-of-Stake consensus mechanism where validators are penalized for downtime but not for signing conflicting blocks. The analyst flags a major security risk. What is the MOST likely risk they identified?
- Centralization of stake
- Long-Range Attack (Correct answer)
- Resource Exhaustion Attack
- 51% Attack
Correct answer: Long-Range Attack
A Long-Range Attack is a significant vulnerability in some PoS systems. An attacker acquires old private keys from former validators and uses them to create a long alternative chain from a much earlier point in the blockchain's history. Without penalties (slashing) for signing conflicting blocks (equivocation), there is little to stop validators on this new fork. The lack of such penalties is a key enabler for this type of attack.
Question 6: An attacker plans to disrupt a Proof-of-Work blockchain by secretly mining a private chain that is longer than the public chain. The attacker includes a transaction sending funds to an exchange on the public chain, waits for the deposit to be confirmed, and then releases their longer private chain which does not contain this transaction, thereby invalidating it. This allows the attacker to keep their original funds. What is this attack called?
- Timejacking Attack
- Finney Attack
- Selfish Mining (Correct answer)
- Race Attack
Correct answer: Selfish Mining
Selfish mining is an attack strategy where a miner or pool finds a block but withholds it from the public network. They continue to mine on their private fork. If they find another block, they extend their private chain. They only release their blocks strategically to earn a larger share of rewards or to invalidate blocks mined by others, as described in the scenario.
CBSE - Certified Blockchain Security Expert Consensus Algorithm Security Questions and Answers 1
A Proof-of-Stake blockchain is experiencing a chain fork.
A validator decides to validate transactions and create blocks on both forked chains simultaneously to maximize their potential rewards, regardless of which chain ultimately becomes canonical.
What specific security vulnerability does this action represent?
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