Question 1: Which feature of blockchain makes historical records tamper-evident?
Correct answer: B. Hash chaining is the structural mechanism that makes blockchain tamper-evident. Each block includes the hash of the previous block — if any past block is modified, its hash changes, which invalidates every block that follows. All nodes can immediately detect the inconsistency. Proof of Work (A) is a consensus mechanism for adding new blocks, not what makes history tamper-evident.
Question 2: What type of attack requires controlling more than 50% of a blockchain network's consensus power?
Correct answer: C. A 51% attack requires controlling more than half the network's consensus power (hash rate in PoW, staked currency in PoS). With majority control, the attacker can override legitimate consensus, approve fraudulent transactions, and potentially double-spend. A double-spend (D) is the typical goal of a 51% attack — not a separate mechanism.
Question 3: An organization wants to track pharmaceutical products from manufacturer to patient, with tamper-evident records that no single participant can alter. Which technology is most appropriate?
Correct answer: B. A permissioned blockchain (private/consortium blockchain) allows known participants (manufacturer, shippers, distributors, pharmacies) to record transactions that no single participant can alter retroactively. The distributed, hash-chained structure provides tamper-evident chain of custody. A centralized database (A, C) still has a central authority that can be hacked or corrupted. Digital certificates (D) authenticate participants but don't provide a distributed tamper-evident ledger.
Question 4: What is a smart contract?
Correct answer: B. A smart contract is code deployed to the blockchain that automatically executes when defined conditions are met — no intermediary needed. The code is transparent, immutable, and enforced by the network. Example: automatic insurance payouts, automatic payment releases, automatic ownership transfers. It is not just a digitally signed document (A) — it is executable code.
Question 5: What is the primary advantage of Proof of Stake over Proof of Work?
Correct answer: C. Proof of Stake selects validators based on staked cryptocurrency — no competitive computation is needed. PoW requires thousands of miners burning massive electricity racing to solve the same puzzle. PoS replaces computational difficulty with economic incentives (lose your stake if you cheat). PoS is NOT immune to 51% attacks (A) — controlling 51% of the staked currency still theoretically enables majority control, just at enormous economic cost.
Matching: Blockchain Concepts
Match each term to its description.
TERM
Distributed Ledger
Proof of Work
Smart Contract
51% Attack
DESCRIPTION
A record replicated across many nodes with no central authority
Consensus via computational puzzle — miners compete to add the next block
Self-executing code on the blockchain that runs automatically when conditions are met
Gaining majority consensus power to override legitimate transactions
Performance Task
A government agency wants to create a tamper-evident, publicly auditable voting system for municipal elections using blockchain. Design the key components and address: voter privacy, vote integrity, auditability, and how you prevent a 51% attack on the voting chain.
Model Answer:
Voter Privacy:
Voters receive anonymous cryptographic tokens (one per voter) from the election authority before voting opens. The token is tied to voter eligibility through a zero-knowledge proof — the voter proves they are eligible without revealing their identity. Votes are cast using these tokens. The vote transaction is recorded on the blockchain: token → candidate (or policy choice). The token cannot be linked back to the voter's identity after issuance. Each token can only be used once (enforced by the blockchain — double-spending a vote is rejected by consensus).
Vote Integrity:
Every vote is a blockchain transaction. Hash chaining makes the ledger tamper-evident — any retroactive modification breaks all subsequent blocks and is immediately detectable. All votes are final once confirmed by the network — no administrator can alter, delete, or add votes. The election smart contract enforces the rules: valid tokens only, one vote per token, votes only accepted during the voting window.
Auditability:
The blockchain is publicly readable. Any observer (candidate, party, journalist, citizen) can download the full blockchain and verify: the total vote count for each option, that no token was used twice, that all votes occurred within the valid voting period, and that the final tally matches the on-chain record. The audit is trustless — no reliance on the election authority's honesty.
Preventing 51% Attack:
Use a large, distributed network for validation — the more validators/miners, the more expensive a majority control attack. For a government election, a permissioned consortium blockchain (with validators from multiple independent parties: federal government, opposition parties, civil society observers) provides accountability without a single party having majority control. Alternatively, use a large public blockchain as the settlement layer — attacking Bitcoin or Ethereum's consensus is effectively impossible. Validator selection can also incorporate geographic distribution requirements to prevent concentration.
Voter Privacy:
Voters receive anonymous cryptographic tokens (one per voter) from the election authority before voting opens. The token is tied to voter eligibility through a zero-knowledge proof — the voter proves they are eligible without revealing their identity. Votes are cast using these tokens. The vote transaction is recorded on the blockchain: token → candidate (or policy choice). The token cannot be linked back to the voter's identity after issuance. Each token can only be used once (enforced by the blockchain — double-spending a vote is rejected by consensus).
Vote Integrity:
Every vote is a blockchain transaction. Hash chaining makes the ledger tamper-evident — any retroactive modification breaks all subsequent blocks and is immediately detectable. All votes are final once confirmed by the network — no administrator can alter, delete, or add votes. The election smart contract enforces the rules: valid tokens only, one vote per token, votes only accepted during the voting window.
Auditability:
The blockchain is publicly readable. Any observer (candidate, party, journalist, citizen) can download the full blockchain and verify: the total vote count for each option, that no token was used twice, that all votes occurred within the valid voting period, and that the final tally matches the on-chain record. The audit is trustless — no reliance on the election authority's honesty.
Preventing 51% Attack:
Use a large, distributed network for validation — the more validators/miners, the more expensive a majority control attack. For a government election, a permissioned consortium blockchain (with validators from multiple independent parties: federal government, opposition parties, civil society observers) provides accountability without a single party having majority control. Alternatively, use a large public blockchain as the settlement layer — attacking Bitcoin or Ethereum's consensus is effectively impossible. Validator selection can also incorporate geographic distribution requirements to prevent concentration.