The vulnerability and importance of computers, robots, internet etc, demand the employment of exceedingly reliable methods in the design of secure systems. Security protocols are one of the most important design parameters. History has proven security protocols to be vulnerable even after they enjoyed circumspect design and meticulous review by experts. Plethora of formal systems exist in order to verify a security protocol. We posit that understanding the subtle issues in security protocols is important when designing a formal framework. In particular, we discuss authentication protocols. We mention their characteristics and point out important design features that a security protocol designer should focus in the design process. We also describe the notion of a penetrator and specify his characteristics. Our purpose is to emphasize the design criteria of an authentication protocol, clarify some subtleties, and give suggestions and pointers that may become helpful in the field of design and verification of cryptographic protocols.