Parisa Momeni - Department of Computer Science Shahid Beheshti University, G.C. Tehran, Iran
ZIBA ESLAMI - Cyberspace Research Center Shahid Beheshti University Tehran, Iran

Oblivious transfer (OT) is an important cryptographic protocol in which a sender possesses a database of messages and a receiver queries the sender for a particular member of this database. This is done in such a way that the sender remains oblivious of what is transfered to the receiver. OT with threshold access control (OT-thAC) is a special case of OT in which for each record in a database, the sender associates an attribute-based access control such that the record can only be available to authorized receivers receivers who possess at least a predefined number of attributes. Meanwhile, after executing the protocol, the sender will not learn anything about associated attributes of the receiver. To the best of our knowledge, all existing OT-thAC protocols need a trusted third party called issuer. In this paper, we propose the first issuer-free OT-thAC that is secure against colluding attacks. The proposed scheme is secure in the presence of malicious adversary under Decisional Modified Bilinear Diffe-Hellman (MBDH) and q-Strong Diffie-Hellman (q-SDH) assumptions.

oblivious transfer; threshold access control; ciphertext-policy attribute based encryption