Blockchain Scalability and its Foundations in Distributed Systems | Week 5

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Course Name: Blockchain Scalability and its Foundations in Distributed Systems

Course Link: Blockchain Scalability and its Foundations in Distributed Systems

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Module 5 quiz

Q1. By assuming partial synchrony:
(There may be more than one correct answer)

  • the algorithm knows the bound on the amount of time.
  • the algorithm does not know the bound on the amount of time.
  • every message takes less than a bounded amount of time to reach its destination.
  • every message takes more than a bounded amount of time to reach its destination.

Answer: the algorithm does not know the bound on the amount of time.
every message takes less than a bounded amount of time to reach its destination.

Q2. The misbehavior of a single node can slow down the performance of PBFT, which role does this node play?

  • The leader
  • The backup
  • The follower

Answer: The leader

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q3. Regarding leader-based algorithms like PBFT, which of the following is NOT correct?

  • The set of transactions is proposed by the leader.
  • It is hard for the client to know whether the leader is misbehaving.
  • The process that plays the role of the leader is permanent.

Answer: The process that plays the role of the leader is permanent.

Q4. For a client or wallet application, it is easier to identify and send requests to whom?

  • The process which is the leader.
  • The processes that will run the consensus.

Answer: The processes that will run the consensus.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q5. At large scale, the time to exchange sets of transactions is typically faster for what kind of consensus algorithms?

  • Leaderless
  • Leader-based

Answer: Leaderless

Q6. The time it takes for all processes to be aware of the set of transactions is the same for both leader-based and leaderless algorithms in what kind of network?

  • Small
  • Large

Answer: Small

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q7. Consensus may not scale in a leader-based algorithm for various reasons. One reason is because consensus here does not leverage all the links.
What does this mean?

  • The decided block must be proposed by the leader.
  • The decided block has to be proposed by a non-leader process.

Answer: The decided block must be proposed by the leader.

Q8. As we learned in this course, if consensus was reached by letting a leader impose its block proposals on other processes, the set of committed transactions would not scale with the amount of participants. Thus, the key to scalability in this situation is to decide on a block that grows with the number of participants in the consensus.
Which of the following is characteristic of this scalable consensus?

  • The decided block has to be the block proposed by the leader.
  • The decided block can contain transactions proposed by different nodes.
  • The decided block has to be a block proposed by one node.

Answer: The decided block can contain transactions proposed by different nodes.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q9. Why does the Set Byzantine Consensus accept potentially valid transactions proposed by malicious nodes?

  • Because, due to their validity, these transactions only withdraw from the account of malicious nodes anyway.
  • Because this consensus is designed for non-critical applications where malicious nodes can sometimes steal the assets of other nodes.

Answer: Because, due to their validity, these transactions only withdraw from the account of malicious nodes anyway.

Q10. In Set Byzantine Consensus, which of the three properties of consensus are different from previous Byzantine consensus definition?

  • Validity
  • Agreement
  • Termination

Answer: Validity

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q11. True or False?
DBFT is called “Democratic” because no misbehaving leader can force the protocol to slow down.

  • True
  • False

Answer: True

Q12. True or False?
DBFT invokes multiple instances of a binary Byzantine consensus, each taking a few message delays to terminate. These instances cannot run concurrently for DBFT to terminate quickly.

  • True
  • False

Answer: False

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q13. Red Belly Blockchain is described as a community blockchain. What makes it different from consortium blockchains?

  • The set of validators and consensus participants are pre-determined.
  • The set of validators and consensus participants change at runtime.

Answer: The set of validators and consensus participants change at runtime.

Q14. Red Belly Blockchain is described as a community blockchain. What makes it different from public blockchains?

  • The system incentivizes all participants to mine the same blocks.
  • The system does not incentivize all participants to mine the same blocks.

Answer: The system does not incentivize all participants to mine the same blocks.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q15. Red Belly Blockchain uses verification sharding to ensure that:

  • The storage resource of the processes is not wasted.
  • The CPU resource of the processes is not wasted.
  • The bandwidth of the processes is not wasted.

Answer: The CPU resource of the processes is not wasted.

End of course quiz

Q1. In 1998, Wei Dai sent a message on the cypherpunk mailing list, describing a system called “b-money”. The system Dai described included a significant idea that is being used in blockchain systems today.
What was the significant idea first proposed in the “b-money” system?

  • Application of a chain of hash to guarantee immutability
  • Public keys to represent user accounts
  • A protocol to transfer cryptocurrency in a peer-to-peer fashion without the need of any intermediary

Answer: Public keys to represent user accounts

Q2. What prevents a participant from withdrawing coins from someone else’s account in a blockchain?

  • Participants have to pass a clearance check before they can start issuing transactions.
  • A key, which is private to the owner of the account, must be used in order to sign the transaction.

Answer: A key, which is private to the owner of the account, must be used in order to sign the transaction.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q3. Which of the following is correct regarding the genesis block?

  • It defines the start of the blockchain.
  • It is known only to the miners who create the new block to be appended to the blockchain.
  • It points to other blocks in the blockchain.

Answer: It defines the start of the blockchain.

Q4. What can go wrong during the blockchain execution given its distributed nature?

  • Two nodes may not have the same genesis block.
  • Two nodes may disagree on a block at a given index higher than the genesis block.

Answer: Two nodes may disagree on a block at a given index higher than the genesis block.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q5. Which of the following features are present in a DAG but not in a chain?

  • Forks
  • Genesis block
  • Appended block

Answer: Forks

Q6. After some time, one could expect that Bob and Carole exchange their views so that they can detect disagreement. What could prevent them from detecting this disagreement?

  • Not knowing which blocks to compare
  • A network delay
  • A conflicting transaction

Answer: A network delay

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q7. What type of failure is most serious?

  • Crash failure
  • Arbitrary failure

Answer: Arbitrary failure

Q8. Which of the following statements best defines the property of termination?

  • The value decided in consensus cannot come out of thin air.
  • Every correct node eventually decides.
  • No two correct nodes would decide differently.

Answer: Every correct node eventually decides.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q9. According to Fisher, Lynch and Patterson (1985), there is no solution to the consensus problem among n nodes if:
(There may be more than one correct answer)

  • there is at least one failure.
  • there is no fixed upper-bound on the time it takes to deliver a message.
  • there is no failure.

Answer: there is at least one failure.
there is no fixed upper-bound on the time it takes to deliver a message.

Q10. A new transaction has been propagated and a miner has created a new block. Which of the following would a miner include in their newly created block?
(There may be more than one correct answer)

  • Some of the transactions the miner has received previously
  • Their private key
  • The solution to the cryptopuzzle

Answer: Some of the transactions the miner has received previously
The solution to the cryptopuzzle

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q11. The diagram above shows the potential of which of the following attacks?

  • 51% attack
  • Sybil attack
  • Rosenfeld’s attack

Answer: 51% attack

Q12. True or False?
In the context of propagation delay of blocks, a Byzantine miner must own more than half of the total mining power of the system for this attack to succeed.

  • True
  • False

Answer: False

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q13. What is the time complexity of the consensus algorithm that works in the absence of failures?

  • O(1)
  • O(n)

Answer: O(1)

Q14. What is the message complexity of the crash tolerant consensus algorithm?

  • O(f2)
  • O(fn2)
  • O(n2)
  • O(nf2)

Answer: O(fn2)

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q15. What is the time complexity of the crash tolerant consensus algorithm?

  • O(n)
  • O(f)
  • O(fn)
  • O(f+n)

Answer: O(f)

Q16. What is the communication complexity, expressed in bits, of the Exponential Information Gathering (EIG) Byzantine fault tolerant algorithm, with b the maximum size in bits of a message?

  • O(bn(f+1))
  • O(b(f+1)n)
  • O(bnf+1)
  • O(bn2)

Answer: O(bnf+1)

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q17. What is the number n of nodes that should run a consensus algorithm to tolerate f Byzantine nodes in a synchronous network (without authentication)?

  • f+1
  • 2f+1
  • 3f+1

Answer: 3f+1

Q18. What does the communication complexity of the consensus algorithms generally indicate?

  • The solutions to the consensus in the presence or absence of failures have the same communication cost.
  • The cost of solving consensus while tolerating failures depends on the severity of the type of failures.
  • Solving consensus that tolerates arbitrary failures involves as much effort as solving consensus that tolerates crash failures.

Answer: The cost of solving consensus while tolerating failures depends on the severity of the type of failures.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q19. True or False?
Synchronization is a realistic assumption when considering that communication occurs over the Internet.

  • True
  • False

Answer: False

Q20. In classic blockchains, the chain can fork. What is the recommendation for users to consider that a transaction is committed?

  • To wait for 1 hour
  • To wait until they can see a number k of blocks are appended to the blockchain the block containing this transaction and where k is a parameter of the blockchain.
  • To make sure this transaction is part of the first block observed at a particular index

Answer: To wait until they can see a number k of blocks are appended to the blockchain the block containing this transaction and where k is a parameter of the blockchain.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q21. True or False?
It is possible for a Byzantine node to double spend without owning a large portion of the mining power in proof of work blockchains.

  • True
  • False

Answer: True

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q22. Let E3 be a set of edges that separates the communication graph in two subgraphs G1 and G2 with the same cumulative mining power. What should an attacker with a low mining power do to execute the Balance Attack?

  • Mine blocks in both subgraphs
  • Delay messages in E3
  • Delay messages in E1
  • Mine blocks in one subgraph only
  • Issue a transaction in the subgraph where the attacker did not mine blocks

Answer: Delay messages in E3
Mine blocks in one subgraph only
Issue a transaction in the subgraph where the attacker did not mine blocks

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q23. Network attacks that enable to intercept messages in the network are often called what?

  • Sybil Attack
  • Man-in-the-middle attack
  • Rosenfeld’s attack
  • 51% attack

Answer: Man-in-the-middle attack

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q24. True or False
Ethereum is vulnerable to man-in-the-middle attacks when run in a consortium environment.

  • True
  • False

Answer: True

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q25. At large scale, the time to exchange sets of transactions is typically faster for what kind of consensus algorithms?

  • Leaderless
  • Leader-based

Answer: Leaderless

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q26. In Set Byzantine Consensus, which of the three properties of consensus are different from previous Byzantine consensus definition?

  • Validity
  • Agreement
  • Termination

Answer: Validity

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q27. Red Belly Blockchain is described as a community blockchain. What makes it different from consortium blockchains?

  • The set of validators and consensus participants are pre-determined.
  • The set of validators and consensus participants change at runtime.

Answer: The set of validators and consensus participants change at runtime.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q28. Red Belly Blockchain is described as a community blockchain. What makes it different from public blockchains?

  • The system incentivizes all participants to mine the same blocks.
  • The system does not incentivize all participants to mine the same blocks.

Answer: The system does not incentivize all participants to mine the same blocks.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q29. In Red Belly Blockchain, as more nodes join the set of participants, what generally happens to performance?

  • It increases
  • It decreases

Answer: It increases

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

Q30. Red Belly Blockchain uses verification sharding to ensure that:

  • The storage resource of the processes is not wasted.
  • The CPU resource of the processes is not wasted.
  • The bandwidth of the processes is not wasted.

Answer: The CPU resource of the processes is not wasted.

These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz

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These are Blockchain Scalability and its Foundations in Distributed Systems Week 5 Answer Coursera Quiz
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