# Network Security

Question 1

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

Question 2

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?
• I and II
• I and III
• II and IV
• III and IV

Question 3

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
• P and R only
• Q and R only
• Q and S only
• R and S only

Question 4

A sender is employing public key cryptography to send a secret message to a receiver. Which one of the following statements is TRUE?
• Sender encrypts using receiver’s public key
• Sender encrypts using his own public key
• Receiver decrypts using sender’s public key
• Receiver decrypts using his own public key

Question 5

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 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?
• A
• B
• C
• D

Question 6

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?
• Both are false
• Statement (i) is true and the other is false
• Statement (ii) is true and the other is false
• Both are true

Question 7

Exponentiation is a heavily used operation in public key cryptography. Which of the following options is the tightest upper bound on the number of multiplications required to compute bn mod m,0≤b,n≤m ?
• O(logn)
• O(√n)
• O(n/logn)
• O(n)

Question 8

The total number of keys required for a set of n individuals to be able to communicate with each other using secret key and public key crypto-systems, respectively are:
• n(n-1) and 2n
• 2n and ((n(n - 1))/2)
• ((n(n - 1))/2) and 2n
• ((n(n - 1))/2) and n

Question 9

S1- MD5 is vulnerable to the Birthday attack.
S2- traceroute uses the "Destination port unreachable" ICMP error message.
S3- 3DES is a type of Public Key Encryption Algorithm.
S4- IPv6 has a 40-byte fixed header size(without Options fields).
How many of the above statements are true?
• 2
• 3
• 4
• None of these

Question 10

In a RSA cryptosystem, a participant A uses two prime numbers p=13 and q=11 to generate his public and private keys. If the public key of A is 37, then the private key of A is _____.
• 13
• 35
• 17
• 11

There are 40 questions to complete.

Last Updated :
Take a part in the ongoing discussion