The methods of cryptography can be successful only if they are properly incorporated into an application. A cryptographic technique can be unbreakable as such, and yet could still be insecure if it is used unwisely. This observation motivates the subject of security protocols. A security protocol consists of a formal method of using a cryptographic technique in a practical application. Without a well-defined security protocol, a perfectly secure cryptographic technique could be compromised by its use.

We do not draw a sharp distinction between the topic of secrecy and the topic of security. The two are closely related, and it is not helpful to attempt to draw a sharp line between them. In this context, a primitive is a basic method of obtaining secrecy and a protocol is a body of rules for using that primitive.

In this chapter we will also study other forms of information protection such as identification and secret sharing that can be regarded as topics closely related to security protocols. The topic of identification deals with methods for establishing the identity of the source of a transmission. This is a subtle and difficult topic because the very concept of identity is vague and subjective. The identity of an isolated individual or device may be self-defined and subject to no outside verification. That identity is useful only to establish that all messages in a sequence of messages are from the same entity.