Feebly secure cryptographic primitives

In 1992, A. Hiltgen [9] provided first constructions of provably (slightly) secure cryptographic primitives, namely feebly one-way functions. These functions are provably harder to invert than to compute, but the complexity (viewed as the circuit complexity over circuits with arbitrary binary gates) is amplified only by a constant factor (in Hiltgen's works, the factor approaches $2$). In traditional cryptography, one-way functions are the basic primitive of private-key schemes, while public-key schemes are constructed using trapdoor functions. We continue Hiltgen's work by providing examples of feebly secure trapdoor functions where the adversary is guaranteed to spend more time than honest participants (also by a constant factor). We give both a (simpler) linear and a (better) non-linear construction.