Recap of Symmetric Encryption

Summary

In this chapter, we have concentrated on symmetric encryption algorithms. We looked at the two major classes of symmetric encryption algorithms, namely stream ciphers and block ciphers. Simple stream ciphers are fast and do not propagate errors, making them suitable for poor-quality channels and for applications where errors are intolerable. Block ciphers do propagate errors (to a limited extent) but are quite flexible and can be used in different ways to provide different security properties, even to achieve some of the benefits of stream ciphers in some cases. In particular, we looked at two influential block ciphers, DES and its ‘successor’ AES. We then discussed different modes of operation of a block cipher and their different properties.

Perhaps the most important two messages to take away from this chapter are the following:

• Stream ciphers and block ciphers are different types of symmetric encryption algorithms. They have slightly different properties and are therefore suitable for different applications.

• The properties of cryptographic algorithms are not only affected by algorithm design but also by how the algorithms are used. Different modes of operation can significantly change the properties of a block cipher.