Simulatable Adaptive Oblivious Transfer

We study an

adaptive

variant of oblivious transfer in which a sender has

N

messages, of which a receiver can adaptively choose to receive

k

one-after-the-other, in such a way that (a) the sender learns nothing about the receiver’s selections, and (b) the receiver only learns about the

k

requested messages. We propose two practical protocols for this primitive that achieve a stronger security notion than previous schemes with comparable efficiency. In particular, by requiring full simulatability for both sender and receiver security, our notion prohibits a subtle selective-failure attack not addressed by the security notions achieved by previous practical schemes.

Our first protocol is a very efficient generic construction from unique blind signatures in the random oracle model. The second construction does not assume random oracles, but achieves remarkable efficiency with only a constant number of group elements sent during each transfer. This second construction uses novel techniques for building efficient simulatable protocols.