Paper 2013/063

A Verifiable 1-out-of-n Distributed Oblivious Transfer Protocol

Christian L. F. Corniaux and Hossein Ghodosi

Abstract

In the various 1-out-of-$n$ distributed oblivious transfer protocols (DOT) designed in an unconditionally secure environment, a receiver contacts $k$ out of $m$ servers to obtain one of the $n$ secrets held by a sender. After a protocol has been executed, the sender has no information on the choice of the receiver and the receiver has no information on the secrets she did not obtain. Likewise, a coalition of $k - 1$ servers is unable to infer any information, neither on the sender's secrets, nor on the receiver's choice. These protocols are based on a semi-honest model: no mechanism prevents a group of malicious servers from disrupting the protocol such that the secret obtained by the receiver does not correspond to the chosen secret. Actually, to verify the information transmitted by the servers seems to require some properties difficult to reconcile: on one hand the receiver has to collect more information from the servers to discard the incorrect data generated by the malicious servers; on the other hand, if the receiver is allowed to gather more information from the servers, the sender's security may be compromised. We study the first unconditionally secure DOT protocol in the presence of an active adversary who may corrupt up to $k - 1$ servers. In addition to the active adversary, we also assume that the sender may (passively) corrupt up to $k - 1$ servers to learn the choice of the receiver. Similarly, the receiver may (passively) corrupt up to $k - 1$ servers to learn more than the chosen secret. However, we assume that the sender, receiver, and active adversary do not collaborate with each other. Our DOT protocol allows the receiver to contact $4k - 3$ servers to obtain one secret, while the required security is maintained.