We describe a protocol design process, and illustrate its use by creating ATSPEC, an Authentication Test-based Secure Protocol for Electronic Commerce Transactions. The design process is organized around the authentication tests, a method for protocol verification based on the strand space theory. The authentication tests dictate how randomly generated values such as nonces may be combined with encryption to achieve authentication and freshness.

ATSPEC offers functionality and security guarantees akin to the purchase request, payment authorization, and payment capture phases of SET, the secure electronic transaction standard created by the major credit card firms.