Secret Key Cryptography

12 Principles for secure encryption

This chapter covers

• Five principles for secure encryption
• Large blocks and long keys
• Confusion, or non-linearity
• Diffusion and saturation

Let’s pull together everything we learned in chapter 11. In sections 12.1 to 12.5 we will distill the 5 underlying principles that make a block cipher secure. One hallmark of a secure block cipher is that changing any bit in the key or any bit in the plaintext will cause about 50% of the bits in the ciphertext block to change, preferably in a random-looking pattern. Changing any other bit also will cause about 50% of the bits in the ciphertext block to change, but in a different pattern. Let’s call this the Fifty-Fifty property. This chapter will describe how to make that ...