In the world of communications, assurance is sought that (1) a message is not accidentally or deliberately modified in transit by replacement, insertion, or deletion; (2) the message is coming from the source from which it claims to come; (3) the message is protected against unauthorized individuals reading information that is supposed to be kept private; and (4) there is protection against an individual denying that the individual sent or received a message. These assurances are provided through the use of security mechanisms. Chapters IV, V, VI, and VII discuss security mechanisms such as confidentiality, integrity, and access authentication that are used to implement the security services listed above. This chapter covers two types of symmetric encryption: stream ciphers and block ciphers. The theory behind using shift registers as stream ciphers, as well as the DES and the Advanced Encryption Standard (AES), are also covered in this chapter.