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- **fileName**: study_security_l4
- **Created on**: 2025-06-02 21:11:28

Block Ciphers & DES Made Simple

What is a Block Cipher?

A secret code machine that works on fixed-size data chunks
(blocks). Example: DES works on 64-bit blocks (8 letters).

Block vs Stream Ciphers

• Block: Encrypts in chunks (like cutting cake into slices)
  Ex: DES, AES
• Stream: Encrypts bit-by-bit (like pouring syrup)
  Ex: RC4

How DES Works (Simple Steps)

1. Initial Mix-Up: Shuffle the message (permutation)
2. 16 Mixing Rounds:
   • Cut message in half (Left & Right)
   • Right half gets stretched, mixed with mini-key
   • Pass through secret boxes (S-boxes)
   • Shuffle again (P-box)
   • Left half gets mixed with result
   • Swap halves
3. Final Mix-Up: One last shuffle

Key Features:

• S-boxes: Secret bit changers
• P-boxes: Bit rearrangers
• Confusion: Makes key-text relationship messy
• Diffusion: Spreads plaintext effects

S-DES Example Walkthrough

Problem:

Encrypt 00101000 with key 1100011110

Key Making:

1. P10(key): 1100011110 → 0011001111
2. Split: 00110 | 01111
3. Shift left 1: 01100 | 11110
4. P8 → K1: 11101001
5. Shift left 2: 10001 | 11011
6. P8 → K2: 10100111

Encryption:

1. IP: 00101000 → 00100010
2. Round 1:
   • Right: 0010 → 00010100 (expanded)
   • XOR K1: 11111101
   • S-box → 1000
   • P4 → 0010
   • New left: 0011 (original XOR result)
   • Swap: 0010 0011
3. Round 2 (with K2) → Ciphertext: 10001010

Review Questions

1. Block vs Stream Cipher?

   • Block: Encrypts chunks (DES)
   • Stream: Encrypts continuously (RC4)

2. Shannon's Theory?
   Needs both:

   • Confusion (messy key-text)
   • Diffusion (spread effects)

3. Feistel Cipher?
   Split → Mix right → Swap → Repeat

4. DES Steps?
   Shuffle → 16 mix rounds → Final shuffle

5. S-DES Practice:
   For 11110011 with 1010000010:

   • Make keys same way
   • Do 2 encryption rounds

Key Tips:

• DES: Old but important standard

• S-DES: Simplified learning version

• XOR: 1+1=0, 0+1=1 (no carry)

• Always use slide tables for permutations

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