Contract Signing, Optimism, and Advantage

Abstract. A contract signing protocol lets two parties exchange digital signatures on a pre-agreed text. Optimistic contract signing protocols enable the signers to do so without invoking a trusted third party. However, an adjudicating third party remains available should one or both signers seek timely resolution. We analyze optimistic contract signing protocols using a game-theoretic approach and prove a fundamental impossibility result: in any fair, optimistic, timely protocol, an optimistic player yields an advantage to the opponent. The proof relies on a careful characterization of optimistic play that postpones communication to the third party. Since advantage cannot be completely eliminated from optimistic protocols, we argue that the strongest property attainable is the absence of provable advantage, i.e., abuse-freeness in the sense of Garay-Jakobsson-MacKenzie.

• Type: Conference
• Year: 2003
• Where: CONCUR
• Authors: Rohit Chadha, John C. Mitchell, Andre Scedrov, Vitaly Shmatikov
• Keywords: CONCUR 2003, Contract Signing Protocols, Optimistic Contract Signing, Optimistic Protocols