A Fair Protocol for Signing Contracts

Michael Ben-Or*, Oded Goldreich, Silvio Micali, Ronald L. Rivest

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

213 Scopus citations

Abstract

Two parties, A and B, want to sign a contract C over a communication network. To do so, they must “simultaneously” exchange their commitments to C. Since simultaneous exchange is usually impossible in practice, protocols are needed to approximate simultaneity by exchanging partial commitments in piece by piece manner. During such a protocol, one party or another may have a slight advantage; a “fair” protocol keeps this advantage within acceptable limits. We present a new protocol that is fair in the sense that, at any stage in its execution, the conditional probability that one party cannot commit both parties to the contract given that the other party can, is close to zero. This is true even if A and B have vastly different computing powers, and is proved under very weak cryptographic assumptions. Our protocol has the following additional properties: During the procedure the parties exchange probabilistic options for committing both parties to the contract; • the protocol never terminates in an asymmetric situation where party A knows that party B is committed to the contract while he is not; • the protocol makes use of a weak form of a third party (judge). If both A and B are honest, the judge will never be called upon. Otherwise, the judge rules by performing a simple computation. No bookkeeping is required of the judge.

Original languageAmerican English
Pages (from-to)40-46
Number of pages7
JournalIEEE Transactions on Information Theory
Volume36
Issue number1
DOIs
StatePublished - Jan 1990
Externally publishedYes

Bibliographical note

Funding Information:
Manuscript received August 8, 1986; revised January 5, 1989. This work was supported in part by a Weizmann Postdoctoral Fellowship, in part by the National Science Foundation under Grants DCR-8413577 and MCS80-06938, and in part by an IBM Faculty Development Award. The material in this paper was partially presented at the 12th ICALP, Greece. 1985. M. Ben-Or was with the Laboratory for Computer Science, Massachusetts Institute of Technology, Cambridge, MA. He is now with the Institute of Mathematics and Computer Science, Hebrew University, Jerusalem, Israel. 0. Goldreich was with the Laboratory for Computer Science, Massachusetts Institute of Technology, Cambridge, MA. He is now with the Computer Science Department, Technion-Israel Institute of Technology, Haifa 32000, Israel. S. Micah and R. L. &vest are with the Laboratory for Computer Science, Massachusetts Institute of Technology, 545 Technology Square, Cambridge, MA 02139. IEEE Log Number 8933051.

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