Network Security

Question 1
Using public key cryptography, X adds a digital signature \\sigma to message M, encrypts < M, \\sigma >, and sends it to Y, where it is decrypted. Which one of the following sequences of keys is used for the operations?
Cross
Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key
Cross
Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key
Cross
Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key
Tick
Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’s public key


Question 2
Using public key cryptography, X adds a digital signature \\sigma to message M, encrypts < M, \\sigma >, and sends it to Y, where it is decrypted. Which one of the following sequences of keys is used for the operations?
Cross
Encryption: X’s private key followed by Y’s private key; Decryption: X’s public key followed by Y’s public key
Cross
Encryption: X’s private key followed by Y’s public key; Decryption: X’s public key followed by Y’s private key
Cross
Encryption: X’s public key followed by Y’s private key; Decryption: Y’s public key followed by X’s private key
Tick
Encryption: X’s private key followed by Y’s public key; Decryption: Y’s private key followed by X’s public key


Question 3
In the RSA public key cryptosystem, the private and public keys are (e, n) and (d, n) respectively, where n = p*q and p and q are large primes. Besides, n is public and p and q are private. Let M be an integer such that 0 < M < n and f(n) = (p- 1)(q-1). Now consider the following equations.
 
I.  M’= Me mod n
    M = (M’)d mod n 

II.  ed ≡ 1 mod n 

III. ed ≡ 1 mod f(n)

IV. M’= Me mod f(n)
    M = (M’)d mod f(n) 
Which of the above equations correctly represent RSA cryptosystem?
Cross
I and II
Tick
I and III
Cross
II and IV
Cross
III and IV


Question 3-Explanation: 
I is true because below is true in RSA-Cryptosystem.
Encrypted-Text = (Plain-Text)e mod n
Plain-Text = (Encrypted-Text)d mod n 
III is true because below is true
   d-1 = e mod ϕ(n)
OR ed = 1 mod ϕ(n) 
Question 4
Which of the following are used to generate a message digest by the network security protocols?
(P) RSA 
(Q) SHA-1 
(R) DES 
(S) MD5
Cross
P and R only
Cross
Q and R only
Tick
Q and S only
Cross
R and S only


Question 4-Explanation: 
  • RSA – It is an algorithm used to encrypt and decrypt messages.
  • SHA 1 – Secure Hash Algorithm 1, or SHA 1 is a cryptographic hash function. It produces a 160 bit (20 byte) hash value (message digest).
  • DES – Data Encryption Standard, or DES is a symmetric key algorithm for encryption of electronic data.
  • MD5 – Message Digest 5, or MD5 is a widely used cryptographic hash function that produces a 128 bit hash value (message digest).
Q and S i.e SHA 1 and MD5 are used to generate a message digest by the network security protocols. So, C is the correct choice.
Question 5
Suppose that everyone in a group of N people wants to communicate secretly with the N–1 others using symmetric key cryptographic system. The communication between any two persons should not be decodable by the others in the group. The number of keys required in the system as a whole to satisfy the confidentiality requirement is
Cross
2N
Cross
N(N – 1)
Tick
N(N – 1)/2
Cross
(N – 1)2


Question 5-Explanation: 
In Symmetric Key Cryptography, access of key is with both the parties. It implies every person needs to communicate N-1 other users using different keys i.e 1+2+3...N-2+N-1 This is like number of edges needed in a complete graph with N vertices is N(N-1)/2. Answer is therefore C
Question 6
A sender is employing public key cryptography to send a secret message to a receiver. Which one of the following statements is TRUE?
Tick
Sender encrypts using receiver’s public key
Cross
Sender encrypts using his own public key
Cross
Receiver decrypts using sender’s public key
Cross
Receiver decrypts using his own public key


Question 7
Consider that B wants to send a message m that is digitally signed to A. Let the pair of private and public keys for A and B be denoted gt20 represent the operation of encrypting m with a key Kx and H(m) represent the message digest. Which one of the following indicates the CORRECT way of sending the message m along with the digital signature to A? gt18
Cross
A
Tick
B
Cross
C
Cross
D


Question 7-Explanation: 
Digital signature are electronic signatures which ensures the integrity ,non repudiation and authenticity of message.Message digest is a hash value generated by applying a function on it. Message digest is encrypted using private key of sender ,so it can only be decrypted by public key of sender.This ensures that the message was sent by the known sender. Message digest is sent with the original message to the receiving end,where hash function is used on the original message and the value generated by that is matched with the message digest.This ensures the integrity and thus,that the message was not altered. Digital signature uses private key of the sender to sign digest. So option B is correct as it is encrypting digest of message H(m) using its private key K-B. \"\" This solution is contributed by Shashank Shanker khare.
Question 8
Anarkali digitally signs a message and sends it to Salim. Verification of the signature by Salim requires
Tick
Anarkali’s public key.
Cross
Salim’s public key.
Cross
Salim’s private key.
Cross
Anarkali’s private key.


Question 8-Explanation: 
Sender uses its private key to digitally sign a document. Receiver uses public key of sender to verify. So option A is correct.
Question 9
Consider the following two statements: i. A hash function (these are often used for computing digital signatures) is an injective function. A. encryption technique such as DES performs a permutation on the elements of its input alphabet. Which one of the following options is valid for the above two statements?
Cross
Both are false
Cross
Statement (i) is true and the other is false
Tick
Statement (ii) is true and the other is false
Cross
Both are true


Question 9-Explanation: 
   

Injective Function:

A function F(X) is said to be injective if it has one-to-one mapping. Statement 1: Hash function is an injective function Statement 2: DES Encryption technique performs a permutation on the elements of its input alphabet

1) Generally, a hash function H(X) is mapping from a larger set to a predefined output set For example, let H(X) = (X)%5 The above function H(X) is not injective because Let X1 = 10, X2 = 15 H(10) = H(15) = 0 As the output of H(X1) = H(X2) where X1!= X2 => H(X) is many-to-one function. Statement 1 is false.

2) In DES encryption scheme, it performs P-Box permutation. Statement 1 is false, Statement 2 is true.

This solution is contributed by Anil Saikrishna Devarasetty.

Question 10
The minimum positive integer p such that 3p modulo 17 = 1 is
Cross
5
Cross
8
Cross
12
Tick
16


Question 10-Explanation: 
Either use Fermat\'s Little Theorem or put the value of p and get the answer. Option (D) is correct.
There are 41 questions to complete.

  • Last Updated : 09 Oct, 2019

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