Given a cryptographic protocol, and some assumptions, can we present everything that can happen, subject to these assumptions? The assumptions may include: (i) some behavior assumed to have occurred, (ii) some keys assumed to be uncompromised, and (iii) some values assumed to have been freshly chosen. An object representing these types of information is called a skeleton.

The shapes for a skeleton A are the minimal, essentially different executions that are compatible with the assumptions in A. The set of shapes for an A is frequently but not always finite. Given a finite set of shapes for A, it is evident whether a security goal such as authentication or confidentiality holds for A.

In this paper, we describe a search that finds the shapes, starting from a protocol and a skeleton A. The search is driven by the challenge-response patterns formalized in the strand space authentication tests.