I am wondering what concrete computable functions we know that
- are a permutation over an integer interval of parameterizable size $s$, for relatively small $s$ starting circa $2^{64}$, to perhaps $2^{256}$; without loss of generality we'll translate the interval to $\{0\dots s-1\}$;
- are easily computable in the forward direction, but difficult to compute in the backward direction; I'm interested in maximizing the figure of merit $M$ defined as the ratio of the expected amount of work starting from a random point for one computation in backward direction, to that in forward direction (any necessary pre-computation starting from $s$ is included in said amounts of work).
Obvious remark: the merit $M$ can be no more than $s/2$, which corresponds to computing the inverse permutation by brute-forcing the forward direction. However I would be content with much lesser merit, and wonder what limit there is, be it theoretical or practical.
Any thought, idea, or reference?
Note: I want a permutation for any $s\ge2^{64}$, but a restriction to some mildly sparse subset of values for $s$ is enough, because a well-known technique allows to efficiently turn a permutation $\widehat P$ over an interval of size $\widehat s$ slightly larger than $s$ into the desired permutation $P$ over an interval of size $s$: for $x\in\{0\dots s-1\}$ we obtain $P(x)$ as the first $y_j<s$ with $y_0=\widehat P(x)$ and $y_{j+1}=\widehat P(y_j)$. The merit is not reduced too much: $M\ge s/\widehat s\cdot\widehat M$ holds, at least heuristically and if we ignore the work involved in finding the appropriate $\widehat s$ from $s$.
Side note (not part of the question): I actually want a family of permutations according to some key $K$ and indistinguishable from a random permutation, with some remaining security against inversion even when $K$ is available, as in that earlier question; but that's easily obtainable from a permutation $P$ as in the present question, and a fast keyed permutation (that is, a cipher) $C_K$ over $\{0\dots s-1\}$, by considering $C_K\circ P$; and we can construct $C_K$ from a block cipher with $\lceil\log_2s\rceil$-bit blocks, and the above technique. Also we can sort of stretch the figure of merit $M$ into sizable security against inversion even when $K$ is available, by iterating $C_{K_j}\circ P_j$ with derived keys ${K_j}$ as many times as practical given the performance constraints in the forward direction (with slightly different $P_j$, inasmuch as required to maintain the merit; and possibly lighter $C_{K_j}$).