Blockchain and its Applications Assignment 2

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Blockchain and its Applications Assignment 2 Answers

Blockchain and its Applications Assignment 2 Question 1

Let Bob wants to send a long message to Alice. Alice should be able to confirm that it was indeed sent by Bob, and Bob later cannot deny that he had sent the message. They also want that nobody else should be able to see its content. Alice and Bob plan to use public key Cryptography and cryptographic hashing techniques. Let the key pairs of Alice and Bob are (Pub A,Pri A) and (Pub B, Pri B) respectively. Let E, D and H be encryption, decryption and hash functions, respectively. Let M denote Message and H(M) its digest. Which of the following describes the correct order of steps to be used by Alice to send the digital signed message

i. At Bob: M’ = E(M, KpubA)

ii. At Alice: M = E(M’, KpriA)

iii. Bob sends the message M’ to Alice

iv The signature along with the message is sent to Alice (M,M’)

v. Bob: M’ = E(M, KpriB)

vi. Signing the message with his private key: S = E(H(M), KpriB)

vii. M = E(K’, KpubB)

a. i,iii,ii,v,iv,vii,vi

b. i,ii,iii,iv,v,vi,vii

c. v,vii,ii,i,iii,iv,vi

d. vii,vi,v,iv,iii,ii,i

Blockchain and its Applications Assignment 2 Question 1 Answer

The correct order of steps to be used by Alice to send the digital signed message is: a. i,iii,ii,v,iv,vii,vi.

At Bob’s end, the message M is encrypted with Alice’s public key (KpubA) to form M’. This is step i: M’ = E(M, KpubA).
At Alice’s end, the encrypted message M’ is decrypted with her private key (KpriA) to get the original message M. This is step ii: M = E(M’, KpriA).
Bob sends the encrypted message M’ to Alice. This is step iii.
Bob signs the message with his private key (KpriB) to generate a digital signature S. This is step vi: S = E(H(M), KpriB).
Bob sends the digital signature S and the encrypted message M’ to Alice. This is step iv: (M,M’).
At Alice’s end, the digital signature S is decrypted with Bob’s public key (KpubB) to get the digest of the original message M. This is step vii: M = E(K’, KpubB).
Finally, Alice can compare the decrypted digest with the hash of the original message M to confirm that the message was indeed sent by Bob and has not been tampered with.

Note: The order of steps v and vi is interchangeable.

Blockchain and its Applications Assignment 2 Question 2

Digitally signing transactions by sender in Blockchain does not ensure to solve repudiation/verifiability problems. Is the above statement True or False?

a. True

b. False

Blockchain and its Applications Assignment 2 Question 2 Answer

False. The statement is false. Digitally signing transactions by the sender in Blockchain does indeed help to solve repudiation and verifiability problems.

In Blockchain, transactions are verified by the network nodes before being added to the blockchain. Digital signatures are used to ensure the authenticity of the transactions, so that no one can deny having sent a particular transaction. When a sender signs a transaction, they use their private key to generate a digital signature. This digital signature is then verified by the network nodes using the sender’s public key. If the digital signature is valid, the transaction is deemed to be authentic and is added to the blockchain.

In this way, digital signatures provide a way for the sender to prove that they did indeed send a particular transaction, and for the network to verify the authenticity of the transaction, thus solving the problem of repudiation.

Blockchain and its Applications Assignment 2 Question 3

Which of the following is used to point a block in block chain:

a) Hash Pointer

b) User ID

c) Transaction ID

d) Timestamp

Blockchain and its Applications Assignment 2 Question 3 Answer

The correct answer is: a) Hash Pointer.

A hash pointer is a data structure that contains the hash of a previous block in the blockchain. It is used to point to a specific block in the blockchain, making it possible to trace the history of transactions and prevent tampering with the data. A hash pointer is included in each block of the blockchain, so that each block is linked to the previous block, forming a chain of blocks.

Question 4

Suppose you have 6 data points — 1 to 6. The post-order traversal of the Merkle Tree is given by (here 6 means hash of 6, 43 means the combined hash of 4 and 3, and so on):

a) {12345656, 1234, 5656, 12, 34, 56, 56, 1, 2, 3, 4, 5, 6}

b) {1, 12, 2, 3, 4, 34, 1234, 5, 6, 56, 123456}

c) {1, 2, 12, 3, 4, 34, 1234, 5, 6, 56, 78, 5678, 12345678}

d) {1, 2, 12, 3, 4, 34, 1234, 5, 6, 56, 56, 5656, 12345656}

Question 4 Answer

The answer is d) {1,2,12,3,4,34,1234,5,6,56,56,5656,12345656}

The post-order traversal of a Merkle Tree starts from the leaves and moves upwards.
The leaves of the tree contain the individual data points. In this case, the leaves are 1 to 6.
The non-leaf nodes represent the hash of the combined hashes of their children.
In this example, the first two hashes 12 and 34 are combined to form the hash 1234, which is then combined with the next two hashes 56 and 56 to form the hash 5656.
Finally, the hashes 1234 and 5656 are combined to form the final hash 12345656.
The final hash is the root of the tree and represents the combined hash of all the data points.
The post-order traversal of the tree follows the bottom-up approach and visits all the leaves and then the non-leaf nodes in a left-to-right order.

Question 5

Which of the following is true for using a digital signature in blockchain?

a. To check the validity of the source of a transactions

b. None of the given options

c. It will ensures that no one can deny of their own transaction

d. It supports user authentication

Question 5 Answer

The true statements for using a digital signature in blockchain are:

a. To check the validity of the source of a transactions

c. It will ensures that no one can deny of their own transaction

Question 6

Which are main Consensus Algorithms?

a. Proof of Work

b. Proof of Wager

c. Proof of Stake

d. Proof of Mining

Question 6 Answer

The main consensus algorithms are:

a. Proof of Work (PoW) c. Proof of Stake (PoS)

Proof of Work is a consensus algorithm used in some blockchain networks, such as Bitcoin, to achieve distributed consensus and prevent malicious behavior. In PoW, nodes compete to solve a computational puzzle and the first node to solve it is allowed to create a new block and add it to the chain.

Proof of Stake is a consensus algorithm used in some blockchain networks, such as Ethereum, to achieve distributed consensus and prevent malicious behavior. In PoS, nodes hold a stake in the network and are selected randomly to create new blocks and validate transactions based on the amount of their stake.

“Proof of Wager” and “Proof of Mining” are not commonly used consensus algorithms in blockchain systems.

Question 7

Why is consensus hard in asynchronous system?

I. No notion of global time

II. faults in network

III. nodes may crash/faulty nodes

a. II, III

b. I, II

c. I, III

d. I, II, II

Question 7 Answer

The correct option is: d. I, II, III

Question 8

Liveliness property ensures the output should be produced within a finite time limit?

a. False

b. True

Question 8 Answer

a. False

Liveness property ensures that the system should make progress in a finite amount of time, but it does not guarantee that the output will be produced within a finite time limit. Liveness refers to the ability of the system to make progress, which means that it should eventually produce an output. However, it does not specify a specific time limit for the output to be produced. This property is distinct from safety properties, which ensure that the system satisfies certain constraints, such as consistency and integrity, even in the presence of faults.

Question 9

Paxos consensus support(s) which of the below properties

a. Liveliness

b. Safety

c. Both

d. None of the above

Question 9 Answer

The correct answer is: b. Safety.

Question 10

Which is/are true for Raft consensus?

a. Crash Fault Tolerant

b. Byzantine Fault Tolerant

c. Both

d. None of the above

Question 10 Answer

The correct answer is: a. Crash Fault Tolerant

Raft consensus is a distributed consensus algorithm that is designed to be crash fault tolerant. This means that the system can continue to operate even if one or more of its nodes crashes or fails. The nodes in a Raft-based system communicate with each other to agree on the state of the system and coordinate their actions.

Raft is not Byzantine Fault Tolerant, which means that it cannot handle nodes that are intentionally malicious or sending incorrect messages.

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