Enhancements of Trapdoor Permutations

We take a closer look at several enhancements of the notion of trapdoor permutations. Specifically, we consider the notions of enhanced trapdoor permutation (Goldreich, Foundation of Cryptography: Basic Applications, 2004 ) and doubly enhanced trapdoor permutation (Goldreich, Computational Complexity: A Conceptual Perspective, 2011 ) as well as intermediate notions (Rothblum, A Taxonomy of Enhanced Trapdoor Permutations, 2010 ). These enhancements arose in the study of Oblivious Transfer and NIZK, but they address natural concerns that may arise also in other applications of trapdoor permutations. We clarify why these enhancements are needed in such applications, and show that they actually suffice for these needs.