Invariant generation techniques in cryptographic protocol analysis

The growing interest in the application of formal methods of cryptographic protocol analysis has led to the development of a number of different techniques for generating and describing invariants that are defined in terms of what messages an intruder can and cannot learn. These invariants, which can be used to prove authentication as well as secrecy results, appear to be central to many different tools and techniques. However, since they are usually developed independently for different systems, it is often not easy to see what they have in common with each other than the ones for which they were developed. We attempt to remedy this situation by giving an overview of several of these techniques, discussing their relationships to each other, and developing a simple taxonomy. We also discuss some of the implications for future research.