messages. We propose a general framework for deriving security protocols from simple components, using composition, refinements, and transformations. As a case study, we examine the structure of a family of key exchange protocols that includes Station-To-Station (STS), ISO-9798-3, Just Fast Keying (JFK), IKE and related protocols, deriving all members of the family from two basic protocols. In order to associate formal proofs with protocol derivations, we extend our previous security protocol logic with preconditions, temporal assertions, composition rules, and several other improvements. Using the logic, which we prove is sound with respect to the standard symbolic model of protocol execution and attack (the "Dolev-Yao model"), the security properties of the standard signature based Challenge-Response protocol and the Diffie-Hellman key exchange protocol are established. The ISO-9798-3 protocol is then proved correct by composing the correctness proofs of these two simple protocols. Although our current formal logic is not sufficient to modularly prove security for all of our current protocol derivations, the derivation system provides a framework for further improvements.

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