Authentication Tests

Suppose a principal in a cryptographic protocol creates and transmits a message containing a new value v, which it later receives back in cryptographically altered form. It can conclude that some principal possessing the relevant key has transformed the message containing v. In some circumstances, this must be a regular participant of the protocol, not the penetrator. An inference of this kind is an authentication test. We introduce two main kinds of authentication test. An outgoing test is one in which the new value v is transmitted in encrypted form, and only a regular participant can extract it from that form. An incoming test is one in which v is received back in encrypted form, and only a regular participant can put it in that form. We combine these two tests with a supplementary idea, the unsolicited test, and a related method for checking that certain values remain secret. Together, they determine what authentication properties are achieved by a wide range of cryptographic protocols. In this paper we introduce authentication tests and illustrate their power, giving new and straightforward proofs of security goals for several protocols. We also illustrate how to use the authentication tests as a heuristic for finding attacks against incorrect protocols. Finally, we suggest a protocol design process. We express these ideas in the strand space formalism [21], and prove them correct elsewhere [8].