Ch2_2

Question 1

What is a hash function?

Answer 1

A one-way cryptographic algorithm that maps input data to a fixed-length output (hash value or digest).

Question 2

Does a hash function use a key?

Answer 2

No, hash functions do not use encryption keys.

Question 3

What are uses of hash functions?

Answer 3

Password protection, message authentication, and ensuring data integrity.

Question 4

Can hash values be reversed to get original data?

Answer 4

No, hash functions are one-way and irreversible.

Question 5

Give examples of popular hash functions.

Answer 5

SHA-1, SHA-2, SHA-3, MD5

Question 6

What happens if a small change is made to input data in a hash function?

Answer 6

The output hash changes drastically, indicating loss of integrity.

Question 7

What is a Message Authentication Code (MAC)?

Answer 7

A digest generated from a message and compared at the receiver’s end to ensure it wasn’t altered.

Question 8

What is symmetric encryption?

Answer 8

Encryption where the same key is used for both encryption and decryption.

Question 9

What is the main challenge with symmetric encryption?

Answer 9

Securely exchanging the key between sender and receiver.

Question 10

Name examples of symmetric algorithms.

Answer 10

• Data Encryption Systems (DES)
• 3DES
• RC4
• Advanced Encryption
  Systems (AES).

Question 11

What are the two types of symmetric ciphers?

Answer 11

Block cipher and Stream cipher

Question 12

What is a block cipher?

Answer 12

Encrypts data in fixed-size blocks (e.g., 64 or 128 bits) using padding if needed.

Question 13

What is a stream cipher?

Answer 13

Encrypts data bit-by-bit or byte-by-byte using a keystream.

Question 14

What is a keystream in stream ciphers?

Answer 14

A pseudo-random sequence of bits XORed with the plaintext to produce ciphertext.

Question 15

Examples of stream ciphers?

Answer 15

RC4, Salsa20, ChaCha20, A5

Question 16

What is asymmetric encryption?

Answer 16

Encryption using two different keys: a public key to encrypt and a private key to decrypt.

Question 17

What are examples of asymmetric encryption uses?

Answer 17

Key exchange, digital signatures, and secure communication.

Question 18

What is hybrid encryption?

Answer 18

Combining symmetric and asymmetric encryption for speed and secure key exchange.

Question 19

What is the main drawback of symmetric encryption for many users?

Answer 19

It requires N(N-1)/2 unique keys for N users.

Question 20

What is a cipher mode of operation?

Answer 20

It defines how to apply a block cipher to securely encrypt large amounts of data.

Question 21

What does ECB stand for?

Answer 21

Electronic Code Book

Question 22

How does ECB mode work?

Answer 22

Each block of plaintext is encrypted separately.

Question 23

What is a major drawback of ECB mode?

Answer 23

Identical plaintext blocks result in identical ciphertext blocks.

Question 24

What does CBC stand for?

Answer 24

Cipher Block Chaining

Question 25

How does CBC mode work?

Answer 25

Each plaintext block is XORed with the previous ciphertext block before encryption.

Question 26

What is the advantage of CBC over ECB?

Answer 26

Even identical plaintext blocks result in different ciphertext due to chaining.

Question 27

What is OFB mode?

Answer 27

Output Feedback Mode, a stream cipher mode that XORs a keystream with plaintext.

Question 28

What is an advantage of OFB mode?

Answer 28

Ciphertext blocks are independent, improving error resistance.

Question 29

What is CFB mode?

Answer 29

Cipher Feedback Mode, a stream cipher mode where encryption of previous ciphertext block is XORed with plaintext.

Question 30

What do OFB and CFB have in common?

Answer 30

Both convert block ciphers into stream ciphers and use XOR with a keystream.