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The newcomer to the subject of cryptographic protocols for authentication and key management is likely to be bemused by the sheer variety of techniques and technical background required. But even before this stage is reached a more fundamental question needs to be faced. What are these protocols there for at all? To answer this question it is necessary to provide an understanding of what sets cryptographic protocols apart from other types of protocols. Above all, what are the ‘rules of the game’ when designing or attacking a protocol, and why are techniques for design and analysis of ordinary (non-cryptographic) protocols not sufficient? This chapter provides necessary background material for those readers who are not already familiar with the topic of cryptographic protocols.