Block Stream (week 10)

Question 1

What is the difference between a block cipher and a stream cipher?

Answer 1

Block cipher: Encrypts data in fixed-size blocks. | Stream cipher: Encrypts data one bit/byte at a time.

Question 2

Why are stream ciphers generally faster than block ciphers?

Answer 2

They process data continuously with less overhead.

Question 3

What are the main weaknesses of DES encryption?

Answer 3

Small 56-bit key size; vulnerable to brute-force.

Question 4

How does Triple DES improve upon DES?

Answer 4

Applies DES encryption three times for added security.

Question 5

What is the primary advantage of AES over DES?

Answer 5

AES has larger keys (128, 192, or 256 bits).

Question 6

What are the key differences between ECB and CBC block cipher modes?

Answer 6

ECB encrypts each block independently; CBC chains blocks together for better security.

Question 7

Which block cipher mode introduces feedback to improve security?

Answer 7

Cipher Block Chaining (CBC).

Question 8

Why is ECB mode considered insecure for some data types?

Answer 8

Identical plaintext blocks create identical ciphertext blocks, revealing patterns.

Question 9

What is an Initialization Vector (IV)?

Answer 9

A random value that ensures identical plaintexts produce different ciphertexts.

Question 10

What role does padding play in block cipher encryption?

Answer 10

Fills incomplete blocks to match the required block size.

Question 11

What is a one-time pad (OTP)?

Answer 11

An encryption method using a random key as long as the message.

Question 12

Why is OTP considered theoretically unbreakable?

Answer 12

The key is random, used only once, and kept secret.

Question 13

What condition must be met for OTP to remain secure?

Answer 13

The key must be random, as long as the message, and never reused.

Question 14

How does XOR apply to OTP encryption?

Answer 14

XOR combines the message with the key.

Question 15

Why does OTP require a key as long as the message?

Answer 15

To ensure randomness and eliminate ciphertext patterns.

Question 16

How are random number generators used in cryptography?

Answer 16

They create unpredictable keys, IVs, and salts.

Question 17

What is the difference between a TRNG and a PRNG?

Answer 17

TRNG: Uses physical processes. | PRNG: Uses algorithms.

Question 18

How does the A5/1 cipher use a frame number in GSM encryption?

Answer 18

The frame number is used to generate the keystream.

Question 19

What is the purpose of a keystream in a stream cipher?

Answer 19

Continuously XORs with plaintext for encryption.

Question 20

How is XOR used in both encryption and decryption?

Answer 20

XORing with the same keystream reverses encryption.

Question 21

What is the primary risk of reused keys in XOR encryption?

Answer 21

Reused keys reveal data patterns.

Question 22

How does modular arithmetic relate to XOR encryption?

Answer 22

XOR functions like addition modulo 2.

Question 23

How is text represented in binary for encryption?

Answer 23

Text is converted to ASCII or Unicode values.

Question 24

What is the ASCII representation for the letter ‘a’?

Answer 24

1100001

Question 25

How can XOR encryption result in non-printable characters?

Answer 25

XOR may produce binary values outside the printable range.

Question 26

Why is XOR preferred over AND or OR in stream ciphers?

Answer 26

XOR reveals no relationship between input and output bits.

Question 27

How does GSM encryption secure voice communication?

Answer 27

By XORing voice data with a keystream.

Question 28

What are the key steps in encrypting plaintext with XOR?

Answer 28

Convert text to binary → Generate keystream → XOR bits.

Question 29

What is the primary risk of reusing XOR keys?

Answer 29

Reused keys expose ciphertext patterns.

Question 30

Why does XOR offer a 50% probability for each bit to be 0 or 1?

Answer 30

Each XOR has an equal chance of flipping or retaining bits.

Question 31

What is the relationship between XOR and modular addition?

Answer 31

XOR is equivalent to addition modulo 2.

Question 32

How does combining XOR with a random key improve security?

Answer 32

Ensures ciphertext patterns are unpredictable.

Question 33

What is the primary vulnerability of OTP if the key is reused?

Answer 33

Attackers can compare ciphertexts and reveal patterns.

Question 34

What is the advantage of using a block cipher with CBC mode?

Answer 34

Each ciphertext block depends on the previous block.

Question 35

How does ciphertext chaining improve security?

Answer 35

Identical plaintext blocks produce different ciphertexts.

Question 36

What is the role of a hash function in password storage?

Answer 36

Transforms passwords into fixed-size values for verification.

Question 37

Why are salted hashes preferred for password protection?

Answer 37

Salting adds uniqueness to each hash.

Question 38

How does the shadow file in Linux enhance password security?

Answer 38

Stores hashed passwords with salts in a restricted file.

Question 39

What is the significance of hashing passwords multiple times?

Answer 39

Increases effort required for brute-force attacks.

Question 40

Why are rainbow tables effective against unsalted hash storage?

Answer 40

Precomputed hashes allow fast lookups for common passwords.