Week 1 - Stream + Block Cypher

Question 1

What is a Stream Cypher?

Answer 1

A cypher than encrypts/decrypts data on a stream of data

Question 2

How does a stream cipher perform encryption?

Answer 2

using bitewise operation (XOR) with the data and a key

e.g

D = 1101101101
K = 1100010101

Ek(D) = 0001111000

use same method to decrypt

Question 3

Why use a stream cipher?

Answer 3

generally faster and better suited for applications with continuous data streams.

e.g
- data is transmitted in real-time
- secure communications

Question 4

What is Block cypher?

Answer 4

encrypts/decrypts on a block of data before moving onto next block.

Question 5

How does a block cipher perform encryption?

Answer 5

For Each block:
1. Split into 2
2. Swap halves
3. Apply XOR operation

e.g

1. 1 = 0110 2= 0010 key = 0110
2. 2 = 0010 1 = 0110
3. 0110 0110
4. Ek(P) = 0100 0000

Question 6

What happens when a message is not a multiple of the key?

Answer 6

Padding is added to expand the message length

e.g key is 8 bytes , message is 12 bytes, 4 padding bytes are added

Question 7

Why are block ciphers important?

Answer 7

a secure way to encrypt larger data sets by breaking them into manageable, fixed-size blocks.