Abstract: Oblivious transfer (OT) is a cryptographic primitive used for choice information hiding for the receiver. As a basic tool for high-level multi-party cryptographic protocol construction, it plays an important role in numerous specific applications. In the k-out-of-n OT(OT\+k\-n), the receiver acquires k selections among the n choice in an oblivious manner. Generally, the construction of the OT\+k\-n involves lots of group exponential operations, which brings a heavy burden for embedded devices with limited computational capabilities. With the proliferation of cloud computing, it is feasible to implement complex cryptographic primitives with the support of powerful computing recourse and high-speed dedicated network provided by the cloud service provider (CSP). In this paper, we propose a service-assisted k-out-of-n OT protocol in single server architecture, which outsources the vast majority of exponentiation operations to the cloud. This scheme is constructed with secret sharing and other fundamental public-key primitives, and it achieves provable security on none-collusion semi-honest model under the decisional Diffie-Hellamn (DDH) hard problem; meanwhile it ensures data privacy against the cloud server. Besides, a detailed description of scheme construction and security proof is presented in the context. As a basic cryptographic primitive in cloud environment, the single server-added oblivious transfer protocol will play an important role in designs of general cloud-assisted multi-party computation protocol as well as developments of secure and efficient cloud service software.

Key words: oblivious transfer (OT), outsourcing computing, decisional Diffie-Hellamn (DDH) assumption, semi-honest model, secure multi-party computation