Summary: Since bit and string oblivious transfer and commitment, two primitives of paramount importance in secure two- and multi-party computation, cannot be realized in an unconditionally secure way for both parties from scratch, reductions to weak information-theoretic primitives as well as between different variants of the functionalities are of great interest. In this context, we introduce three independent monotones – quantities that cannot be increased by any protocol – and use them to derive lower bounds on the possibility and efficiency of such reductions. An example is the transition between different versions of oblivious transfer, for which we also propose a new protocol allowing to increase the number of messages the receiver can choose from at the price of a reduction of their length. Our scheme matches the new lower bound and is, therefore, optimal.
For the entire collection see [Zbl 1131.94006].