International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: A Cryptographic Model for Branching Time Security Properties -- the Case of Contract Signing Protocols

V{\'e}ronique Cortier
Ralf Küsters
Bogdan Warinschi
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Abstract: Some cryptographic tasks, such as contract signing and other related tasks, need to ensure complex, branching time security properties. When defining such properties one needs to deal with subtle problems regarding the scheduling of non-deterministic decisions, the delivery of messages sent on resilient (non-adversarially controlled) channels, fair executions (executions where no party, both honest and dishonest, is unreasonably precluded to perform its actions), and defining strategies of adversaries against all possible non-deterministic choices of parties and arbitrary delivery of messages via resilient channels. These problems are typically not addressed in cryptographic models and these models therefore do not suffice to formalize branching time properties, such as those required of contract signing protocols. In this paper, we develop a cryptographic model that deals with all of the above problems. One central feature of our model is a general definition of fair scheduling which not only formalizes fair scheduling of resilient channels but also fair scheduling of actions of honest and dishonest principals. Based on this model and the notion of fair scheduling, we provide a definition of a prominent branching time property of contract signing protocols, namely balance, and give the first \emph{cryptographic} proof that the Asokan-Shoup-Waidner two-party contract signing protocol is balanced.
  title={A Cryptographic Model for Branching Time Security Properties -- the Case of Contract Signing Protocols},
  booktitle={IACR Eprint archive},
  keywords={foundations / contract signing, balance, scheduling},
  note={full version of ESORICS 2007 13693 received 26 Jun 2007, last revised 29 Jun 2007},
  author={V{\'e}ronique Cortier and Ralf Küsters and Bogdan Warinschi},