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TR24-120 | 15th July 2024 07:07
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#### Consequences of Randomized Reductions from SAT to Time-Bounded Kolmogorov Complexity

**Abstract:**
A central open question within meta-complexity is that of NP-hardness of problems such as MCSP and MK$^t$P. Despite a large body of work giving consequences of and barriers for NP-hardness of these problems under (restricted) deterministic reductions, very little is known in the setting of randomized reductions. In this work, we give consequences of randomized NP-hardness reductions for both approximating and exactly computing time-bounded and time-unbounded Kolmogorov complexity.

In the setting of approximate $K^{poly}$ complexity, our results are as follows.

1. Under a derandomization assumption, for any constant $\delta > 0$, if approximating $K^t$ complexity within $n^{\delta}$ additive error is hard for SAT under an honest randomized non-adaptive Turing reduction running in time polynomially less than $t$, then NP = coNP.

2. Under the same assumptions, the worst-case hardness of NP is equivalent to the existence of one-way functions.

Item 1 above may be compared with a recent work of Saks and Santhanam (CCC'22), which makes the same assumptions except with $\omega(\log n)$ additive error, obtaining the conclusion NE = coNE.

In the setting of exact $K^{poly}$ complexity, where the barriers of Item 1 and [Saks, Santhanam; CCC'22] do not apply, we show:

3. If computing $K^t$ complexity is hard for SAT under reductions as in Item 1, then the average-case hardness of NP is equivalent to the existence of one-way functions. That is, ``Pessiland'' is excluded.

Finally, we give consequences of NP-hardness of exact time-unbounded Kolmogorov complexity under randomized reductions.

4. If computing Kolmogorov complexity is hard for SAT under a randomized many-one reduction running in time $t_R$ and with failure probability at most $1/(t_R)^{16}$, then coNP is contained in non-interactive statistical zero-knowledge; thus NP $\subseteq$ coAM. Also, the worst-case hardness of NP is equivalent to the existence of one-way functions.

We further exploit the connection to NISZK along with a previous work of Allender et al. (ITCS'23) to show that hardness of $K$ complexity under randomized many-one reductions is highly robust with respect to failure probability, approximation error, output length, and threshold parameter.