Module 5

Question 1

What are the 3 techniques of block cipher ?

Answer 1

1: substitution
2: permutation
3: looping

Question 2

Why DES have initial and final permutation?

Answer 2

just make reverse engirding less efficient.

Question 3

What is the relationship of initial and final permutation of DES ?

Answer 3

They are inverse of each other.

Question 4

What are the two method to encrypt multiple blocks ?

Answer 4

• Electronic Code Book(ECB)

- Cipher Block Chaining(CBC)

Question 5

What is the problem with ECB ?

Answer 5

Initial PT block can result identical CT block. help hacker reverse engirding the key.

Question 6

what is the IV process in CBC?

Answer 6

Initialization vector: is a random binary stream

Question 7

what is the problem with CBC?

Answer 7

sequential processing

Question 8

what happens when 3-DES’s k1=k2=k3?

Answer 8

it becomes 1-DES

Question 9

Advantage of Stream cipher ?

Answer 9

1. lightweight- mobile system
2. high throughput -real-time system
3. Bit error only affect one bit.

Question 10

Disadvantage of stream cipher ?

Answer 10

1. sync between sender and receiver

2. strength depends on pseudorandom generator

Question 11

How are the PSG are generated?

Answer 11

it use linear feedback shift register(LFSR) to generate