Unit 3: Consensus Algorithms and Bitcoin Introduction - Practice Quiz

1 What is the main goal of consensus in a distributed system?

2 Why is reaching consensus difficult in a distributed network?

3 What does the safety property of a consensus protocol ensure?

4 What does liveness mean in consensus protocol design?

5 Which type of blockchain allows anyone to participate without prior approval?

6 Which fault model assumes that nodes may behave arbitrarily or maliciously?

7 What must participants do in Proof of Work?

8 In Proof of Stake, validator selection is primarily related to what?

9 Which factor is important when selecting a consensus algorithm?

10 Which consensus approach is generally suitable for a network with known and approved participants?

11 What is Bitcoin primarily designed to be?

12 Who introduced Bitcoin in the original white paper?

13 Which key should a Bitcoin owner keep secret?

14 What is a private key used for in a Bitcoin transaction?

15 What does a Bitcoin transaction transfer?

16 What does UTXO stand for in Bitcoin?

17 How are consecutive blocks connected in a blockchain?

18 What is likely to happen if data inside an old block is changed?

19 What is the main role of Bitcoin miners?

20 What value do Bitcoin miners repeatedly change while searching for a valid block hash?

21 Several replicated nodes receive client requests in different orders. What is the main consensus requirement for maintaining a consistent replicated state?

22 A Byzantine fault-tolerant system requires at least replicas to tolerate Byzantine replicas. What is the maximum number of Byzantine faults that 10 replicas can tolerate?

23 A consensus protocol prioritizes safety during a network partition. What behavior should be expected if the separated groups cannot form the required quorum?

24 In a system of 7 replicas, decisions require a quorum of 5 replicas. What is the minimum possible overlap between any two decision quorums?

25 A replica stops responding because of a power failure but never sends incorrect messages. How should this fault be classified?

26 A blockchain permits anyone to join validation without prior approval and uses an economic resource to limit influence. Which classification best applies?

27 In PBFT, a replica has received a valid pre-prepare message but has not yet received enough matching prepare messages. What should it do?

28 A Raft follower has a more up-to-date log than a candidate requesting its vote. Which rule helps prevent the stale candidate from becoming leader?

29 A consortium blockchain has 12 known validators, needs fast finality, and must avoid energy-intensive mining. Which approach is most suitable?

30 A public network allows anonymous nodes to join freely. Which algorithmic approach most directly provides Sybil resistance through computational cost?

31 Two valid Bitcoin branches temporarily exist after miners discover blocks at nearly the same time. Which branch should a node eventually prefer?

32 A wallet displays a balance of 2 BTC formed from three unspent outputs. How is that balance represented on the Bitcoin network?

33 Alice signs a Bitcoin transaction using her private key. What can other nodes verify using the corresponding public key?

34 A user's Bitcoin private key is exposed, but the associated UTXO has not yet been spent. What is the safest immediate response?

35 A Bitcoin transaction consumes an input worth 1.2 BTC and creates outputs worth 0.70 BTC and 0.45 BTC. What transaction fee does it offer?

36 A wallet spends a 1 BTC UTXO, pays 0.30 BTC to a merchant, and assigns a 0.001 BTC fee. What value should the change output contain?

37 An attacker changes a transaction in an old block while leaving the rest of the stored chain unchanged. What is the immediate structural effect?

38 A block contains 1024 transactions in a balanced binary Merkle tree. How many sibling hashes are needed for a standard Merkle inclusion proof?

39 Suppose Bitcoin's mining target is reduced to half its previous value while hash outputs remain uniformly distributed. What happens to mining difficulty and success probability per hash?

40 What distinguishes the coinbase transaction created by a Bitcoin miner from an ordinary transaction?

41 In a fully asynchronous message-passing system with reliable channels, deterministic processes, and the possibility of one crash failure, which requirement cannot be guaranteed for every admissible execution according to the FLP result?

42 A Byzantine consensus protocol has replicas and requires a quorum of votes. What is the key safety property of any two such quorums?

43 A partially synchronous BFT protocol has four replicas and tolerates one Byzantine failure using three-vote commit certificates. During an extended network partition, the replicas split into two groups of two. What should a correctly designed protocol do?

44 A weighted BFT design has total voting weight , Byzantine weight strictly below , and commit quorums with weight strictly above . Why can two conflicting commit quorums not consist solely of Byzantine overlap?

45 A validator sends two correctly signed but conflicting votes for the same consensus height and round. Under which failure model must this behavior be classified?

46 Which statement most accurately distinguishes Nakamoto consensus from classical BFT consensus under their standard assumptions?

47 In Paxos, a proposer completes Phase 1 and receives promises from a majority. Some responses report previously accepted values with different proposal numbers. Which value must the proposer use in Phase 2?

48 Why does Raft's joint-consensus approach require majorities from both the old and new configurations while a membership change is being committed?

49 A permissioned financial network has seven known validators, must tolerate two validators behaving arbitrarily, and requires low-latency finality. Which design best matches these requirements?

50 A public blockchain requires Sybil-resistant validator participation, low energy consumption, and deterministic finality after a quorum certificate under partial synchrony. Which architecture is the best fit?

51 An attacker controls 60% of Bitcoin's hash rate and mines a high-work chain containing a transaction that spends a nonexistent UTXO. How should an honest fully validating node treat that chain?

52 Two otherwise valid Bitcoin branches share the same ancestor. Branch A adds six blocks, each contributing work equivalent to difficulty . Branch B adds five blocks, each contributing work equivalent to difficulty . Which branch is selected?

53 Two ECDSA signatures and on hashes and reuse the same nonce under private key . Assuming all required inverses exist modulo the curve order , which expression recovers ?

54 A compressed secp256k1 public key stores an -coordinate and a prefix indicating whether is even or odd. Why is this sufficient to reconstruct the full public key?

55 A mempool contains transaction spending UTXO and child transaction spending an output of . A valid replace-by-fee transaction spending replaces . What happens to ?

56 Suppose a native SegWit transaction's witness is changed to a different witness that still satisfies the spending conditions. Before inclusion in a block, which identifier behavior is expected?

57 An SPV client verifies a transaction's Merkle path to a block header and verifies that the header belongs to the highest-chainwork header chain. What has the client still not independently established?

58 Transaction was confirmed on a branch that is later disconnected during a reorganization. The new active branch contains a valid conflicting transaction that spends the same input. What is the resulting status of ?

59 Ignoring timestamp manipulation and assuming the adjustment clamp is not reached, a Bitcoin difficulty period takes 7 days instead of the expected 14 days. If the old target was , what are the approximate new target and difficulty?

60 A miner has exhausted all values of the block header nonce without finding a hash below the target. Which action provides a fresh search space while preserving the candidate block's general transaction set?