Lecture on Feistel Cipher Structure

Learning Outcomes

• Understand the Feistel Cipher structure.
• Know the various design features of the Feistel structure design.

Feistel Cipher Structure Overview

Encryption Process

• Input: Plain text
• Output: Cipher text
• **Process: **
  • Plain text is split into two parts: Left-hand side (LHS) and Right-hand side (RHS).
  • If plain text size is 64 bits, LHS is 32 bits, RHS is 32 bits.
  • RHS is processed through a function with a key (K1), and its output is XORed with LHS.
  • The result becomes RHS for the next round, and the original RHS becomes LHS.
  • Performed over 16 rounds with varying keys (K1 to K16).
  • Final step involves a swap operation between LHS and RHS.

Decryption Process

• Input: Cipher text
• Process:
  • Reverse of encryption: Starts with cipher text and uses keys in reverse order (K16 to K1).
  • Involves the swap operation to retrieve the original plain text.

Key Concepts

Round Function

• Key Components: Substitution and Permutation
• Responsible for providing confusion and diffusion properties to the cipher text.

Design Features of Feistel Structure

1. Block Size

• Determines how many bits can be encrypted/decrypted at a time.
• Examples: DES uses 64-bit block size; AES uses 128-bit block size.

2. Key Size

• Larger key size offers better security but may reduce encryption/decryption speed.

3. Number of Rounds

• More rounds generally increase security.
• Symmetric encryption uses the same key for encryption and decryption.

4. Sub-key Generation Algorithm

• Generates round keys from the original key.
• Needs to be complex to prevent easy cryptanalysis.

5. Round Function

• Should be complex to ensure confusion and diffusion.

6. Fast Encryption and Decryption

• The speed of the algorithm is crucial for practical applications.
• Needs to be fast even with complex designs and multiple rounds.

7. Ease of Analysis

• Difficult for cryptanalysts to analyze and break.
• Should be complex yet explainable to ensure strength.

Conclusion

• The Feistel structure forms the basis for block ciphers like DES and AES.
• Understanding its design features helps in comprehending block cipher functionality.
• The lecture provided insights into encryption/decryption processes and the importance of each design feature in maintaining security.