Methodical use of cryptographic transformations in authentication protocols

Design of cryptographic protocols for authentication and key management is known to be a difficult problem. Although much research has been devoted to analysis techniques there remains a lack of basic design principles. In the paper a common method of protocol design is identified which contributes to protocol problems in a number of ways. This is the practice of encrypting all relevant fields using a reversible cryptographic transformation. A new design principle and a complementary notation are introduced which help protocol designers to identify what form of encryption is really required. Several examples are used to illustrate the problems and to show how the design principle and notation may be used in practice