Design of cryptographic protocols especially authentication protocols remains error-prone, even for experts in this area. Protocol engineering is a new notion introduced in this paper for cryptographic protocol design, which is derived from software engineering idea. We present and illustrate protocol engineering principles in three groups: cryptographic protocol security requirements analysis principles, detailed protocol design principles and provable security principles. Furthermore, we illustrate that some of the well-known Abadi and Needham?s principles are ambiguous. This paper is useful in that it regards cryptographic protocol design as system engineering, hence it can efficiently indicate implicit assumptions behind cryptographic protocol design, and present operational principles on uncovering these subtleties. Although our principles are informal, but they are practical, and we believe that they will benefit other researchers.