# Blockchain and its Applications | Week 2

Session: JAN-APR 2024

Course Name: Blockchain and its Applications

#### Q1. Alice employs the RSA cryptosystem with the prime numbers p = 7 and q = 17 to derive her public and private keys. Given that Alice’s public key is 11, her corresponding private key is __________.

Q2. Bob wishes to send a lengthy message to Alice with the requirement that Alice can verify its origin and Bob cannot later disown the message. They also want to ensure the confidentiality of the message. Alice and Bob decide to employ public key cryptography and cryptographic hashing techniques. Let the key pairs for Alice and Bob be (Pub A, Pri A) and (Pub B, Pri B) respectively, and let E, D, and H denote the encryption, decryption, and hash functions respectively. M represents the message, and H(M) is its digest. Which of the following outlines the correct sequence of steps for Alice to send the digitally signed message?

i. At Bob: M’ = E(M, K_pubA)
ii. At Alice: M = E(M’, K_priA)
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, К_рів)
vi. Signing the message with his private key: S = E(H(M), К_рів)
vii. M = Е(М’, К_рubв)

a. v, vii, ii, i, iii, iv, vi
b. i, iii, ii, v, iv, vii, vi
c. i, ii, iii, iv, v, vi, vii
d. vii, vi, v, iv, iii, ii, i

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

These are Blockchain and its Applications Assignment 2 Answers

Q3. The act of digitally signing transactions by the sender in Blockchain ensures the resolution of repudiation/verifiability problems.
a. True
b. False

Q4. Which of the following is used to point to a block in the blockchain:
a. Hash Pointer
b. User ID
c. Transaction ID
d. Timestamp

These are Blockchain and its Applications Assignment 2 Answers

Q5. Suppose you have 6 data points – A to F. The post-order traversal of the Merkle Tree is given by (here A means hash of A, DC means the combined hash of D and C, and so on):
a. {ABCDEFEF, ABCD, EFEF, AB, CD, EF, EF, A, B, C, D, E, F}
b. {A, AB, B, C, D, CD, ABCD, E, F, EF, ABCDEF}
c. {A, B, AB, C, D, CD, ABCD, E, F, EF, GH, EFGH, ABCDEFGH}
d. {A, B, AB, C, D, CD, ABCD, E, F, EF, EF, EFEF, ABCDEFEF}

Answer: d. {A, B, AB, C, D, CD, ABCD, E, F, EF, EF, EFEF, ABCDEFEF}

Q6. 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 above.
c. It will ensures that no one can deny of their own transaction
d. It supports user authentication

These are Blockchain and its Applications Assignment 2 Answers

Q7. Which are the main Consensus Algorithms?
a. Proof of Work
b. Proof of Wager
с. Proof of Stake
d. Proof of Mining

Q8. Why is consensus hard in an asynchronous system?
I. No notion of global time
II. faults in network
IlI. nodes may crash/ faulty nodes

a. II, III
b. I, II
с. I, III
d. I, II, II

These are Blockchain and its Applications Assignment 2 Answers

Q9. The Liveliness property ensures the output should be produced within a finite time limit?
a. False
b. True

Q10. Paxos consensus support(s) which of the below properties
a. Liveliness
b. Safety
c. Both
d. None of the above

These are Blockchain and its Applications Assignment 2 Answers

Session: JAN-APR 2022

Course Name: Blockchain and its Applications

#### Q1. 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 be (Pub A, Pri A) and (Pub B, Pri B), respectively.

Let E, D and H be the encryption, decryption and hash functions, respectively. Let M denote the Message and H(M) its digest. Which of the following describes the correct order of steps to be used by Alice to send the digitally 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(M’, 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

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

These are Blockchain and its Applications Assignment 2 Answers

Q2. 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

These are Blockchain and its Applications Assignment 2 Answers

Q3. Which of the following is used to point a block in blockchain :
a. Hash Pointer
b. User ID
c. Transaction ID
d. Timestamp

Q4. Suppose you have 6 data points — 1 to 6. The post-order traversal of the Merkle Tree is given by (here 8 means hash of 8, 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}

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

These are Blockchain and its Applications Assignment 2 Answers

Q5. 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

Q6. Which are the main Consensus Algorithms?
a. Proof of Work
b. Proof of Wager
c. Proof of Stake
d. Proof of Mining

These are Blockchain and its Applications Assignment 2 Answers

Q7. 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

These are Blockchain and its Applications Assignment 2 Answers

Q8. Liveliness property ensures the output should be produced within a finite time limit?
a. False
b. True

These are Blockchain and its Applications Assignment 2 Answers

Q9. Paxos consensus support(s) which of the below properties
a. Liveliness
b. Safety
c. Both
d. None of the above

Q10. Which is/are true for Raft consensus?
a. Crash Fault Tolerant
b. Byzantine Fault Tolerant
c. Both
d. None of the above