# Computer Networks Questions & Answers – Cryptography

This set of Computer Networks Multiple Choice Questions & Answers (MCQs) focuses on “Cryptography”.

**1. Cryptanalysis is used __________**

**a) to find some insecurity in a cryptographic scheme**

b) to increase the speed

c) to encrypt the data

d) to make new ciphers

**Explanation:** Cryptanalysis is a branch of mathematics in which a cryptographic scheme is purposefully broken in order to uncover faults and weaknesses. It is used to ensure that the scheme is as secure as possible.

**2. Which one of the following is a cryptographic protocol used to secure HTTP connection?**

a) stream control transmission protocol (SCTP)

**b) transport layer security (TLS)**

c) explicit congestion notification (ECN)

d) resource reservation protocol

**Explanation:** To avoid eavesdropping, tampering, and message forging, TLS uses rigorous message authentication and key-material creation. Since the year 1996, it has been in use.

**3. Voice privacy in GSM cellular telephone protocol is provided by _______**

**a) A5/2 cipher**

b) b5/4 cipher

c) b5/6 cipher

d) b5/8 cipher

**Explanation:** The A5/2 cypher was published in 1996 and cryptanalysed within a month of its publication. Its use was phased down in 2006 due to its poor performance.

**4. ElGamal encryption system is __________**

a) symmetric key encryption algorithm

**b) asymmetric key encryption algorithm**

c) not an encryption algorithm

d) block cipher method

**Explanation:** Taher Elgamal created the ELGamal encryption system in 1985, and it uses an asymmetric key technique. PGP and other systems make extensive use of it.

**5. Cryptographic hash function takes an arbitrary block of data and returns _________**

**a) fixed size bit string**

b) variable size bit string

c) both fixed size bit string and variable size bit string

d) variable sized byte string

**Explanation:** Digital signatures and message authentication codes both use cryptographic hash algorithms. The only flaw is that it always returns the same hash value for a message, leaving it vulnerable to attackers attempting to decode and crack the encryption.

**6. In cryptography, what is cipher?**

**a) algorithm for performing encryption and decryption**

b) encrypted message

c) both algorithm for performing encryption and decryption and encrypted message

d) decrypted message

**Explanation:** A cypher is a means for encrypting and decrypting messages as they transit over a network. It is used to increase the message’s secrecy.

**7. In asymmetric key cryptography, the private key is kept by __________**

a) sender

**b) receiver**

c) sender and receiver

d) all the connected devices to the network

**Explanation:** Only the message’s recipient has access to the private key. Its goal is to ensure that the communication can only be decoded by the intended recipient.

**8. Which one of the following algorithm is not used in asymmetric-key cryptography?**

a) rsa algorithm

b) diffie-hellman algorithm

**c) electronic code book algorithm**

d) dsa algorithm

**Explanation**: Each block of text in an encrypted message corresponds to a block of data in the electronic code book algorithm, which is a block cypher method. For block sizes less than 40 bits, it is not possible.

**9. In cryptography, the order of the letters in a message is rearranged by __________**

**a) transpositional ciphers**

b) substitution ciphers

c) both transpositional ciphers and substitution ciphers

d) quadratic ciphers

**Explanation:** The order of letters in a plaintext message is shuffled using a pre-defined mechanism in transposition cyphers. Rail fence cypher and Columnar transposition are two examples of such cyphers.

**10. What is data encryption standard (DES)?**

**a) block cipher**

b) stream cipher

c) bit cipher

d) byte cipher

**Explanation:** DES is a symmetric key block cypher that uses a 64-bit block size and a 64-bit key size. It is vulnerable to some types of attacks, hence it isn’t widely used.

Cryptography, often known as cryptology, is the practise and study of strategies for secure communication in the presence of third parties known as adversaries (from Ancient Greek: kryptós “hidden, secret”; and graphein, “to write,” or -o -logia, “study,” respectively). Cryptography, in general, is concerned with developing and evaluating protocols that prohibit third parties or the general public from accessing private messages; data confidentiality, data integrity, authentication, and non-repudiation are all important parts of modern cryptography.