Key Principles of Block Cipher Design

1. Confusion

• Makes the relationship between key and ciphertext complex
• Achieved using substitution techniques

✔ Goal: Hide key patterns

2. Diffusion

• Spreads the influence of one plaintext bit over many ciphertext bits
• Achieved using permutation

✔ Goal: Hide statistical structure

3. Substitution–Permutation Network (SPN)

• Combines substitution and permutation operations
• Used in modern ciphers like AES

4. Feistel Structure

• Splits data into two halves and processes them in rounds
• Used in DES

5. Multiple Rounds

• Encryption is done in several rounds
• Each round increases security

6. Key Schedule

• Generates different subkeys from the main key