## Cryptology ePrint Archive: Report 2004/016

A Synchronous Model for Multi-Party Computation and the Incompleteness of Oblivious Transfer

Abstract: This work develops a composable notion of security in a synchronous communication network to analyze cryptographic primitives and protocols in a reliable network with guaranteed delivery. In such a synchronous model the abort of protocols must be handled explicitly. It is shown that a version of *global bit commitment* which allows to identify parties that did not give proper input cannot be securely realized with the primitives *oblivious transfer* and *broadcast*. This proves that the primitives oblivious transfer and broadcast are not complete in our synchronous model of security.

In the synchronous model presented ideal functionalities as well as parties can be equipped with a shell'' which can delay communication until the adversary allows delivery or the number of rounds since the shell received the message exceeds a specified threshold. This additionally allows asynchronous specification of ideal functionalities and allows to model a network where messages are not necessarily delivered in the right order. If these latency times are chosen to be infinite the network is no more reliable and becomes completely asynchronous. It is shown that secure protocols in the setting of [Canetti01] or [CLOS02] can be transformed to secure realizations in the new model if latency times are chosen to be infinite.

Category / Keywords: cryptographic protocols / multi-party computations, oblivious transfer