Formal Analysis of GM Multi-party Contract Signing Protocol

Garay and MacKenzie introduced the first abuse- free multi-party contract signing protocol (GM), Chadha et al. proposed its revised version in order to restore fairness. Based on Paulson's inductive approach and the interactive theorem prover Isabelle/HOL, we analyze the above protocols, still discover serious problems with fairness. We give the formal protocol model that includes infinitely many signatories and contract texts signing simultaneously, formalize cryptographic primitives and protocol arithmetic, consider composite attacks of the dishonest signatory and the external intruder, clarify the behavior of dishonest signatories and corrupt signatories, do great work about protocol attack model. Finally, we propose a revision of the GM protocol.