2.3 Symmetric Encryption Schemes

In the following sections we provide basic explanations of the techniques and concepts related to symmetric encryption schemes, as defined earlier. After reviewing the one-time pad and explaining why it is impractical, we address some techniques that are important when designing block ciphers: substitution and permutation and confusion/diffusion. Then we give some examples of block ciphers and a brief explanation of stream ciphers, the most important classes of symmetrical encryption algorithms.