Design and formal verification of a CEM protocol with transparent TTP

In certified email (CEM) protocols, trusted third party (TTP) transparency is an important security requirement which helps to avoid bad publicity as well as protecting individual users’ privacy. Cederquist et al. proposed an optimistic certified email protocol, which employs key chains to reduce the storage requirement of the TTP. We extend their protocol to satisfy the property of TTP transparency, using existing verifiably encrypted signature schemes. An implementation with the scheme based on bilinear pairing makes our extension one of the most efficient CEM protocols satisfying strong fairness, timeliness, and TTP transparency. We formally verify the security requirements of the extended protocol. The properties of fairness, timeliness and effectiveness are checked in the model checker Mocha, and TTP transparency is formalised and analysed using the toolsets µCRL and CADP.