Informally, one-way functions are defined with respect to PTIME algorithms. They are computable in polynomial time but not invertible in average-case polynomial time. The existence of such functions is an important open problem in theoretical computer science.

I'm interested in one-way functions (not necessarily for cryptographic applications) defined with respect to different resource bounds. Such resource bounds could be LOGSPACE or bounded nondeterminism.

Is there a candidate (natural) problem which is one-way with respect to LOGSPACE algorithms? Is there a candidate (natural) problem which is one-way with respect to nondeterministic linear time algorithms ($\text{NTIME(n)}$)?

I'm fine with worst-case hardness of invertiblity with respect to the above resource bounds.

  • $\begingroup$ Have you seen eprint.iacr.org/2013/001.pdf ? The topic of this paper may or may not be exactly relevant to you, but the techniques in the paper (or perhaps even the citations) may lead to something useful. $\endgroup$ Sep 1, 2013 at 10:01
  • $\begingroup$ The abstract does not help but thanks for your help. $\endgroup$ Sep 1, 2013 at 11:49
  • $\begingroup$ Oh well -- I hope the new answer does though. :) $\endgroup$ Sep 1, 2013 at 16:33
  • $\begingroup$ Yep, It does :) $\endgroup$ Sep 1, 2013 at 20:39

1 Answer 1


Regarding log-space: Several candidate one-way functions are computable in log-space or below (and are supposedly secure even against poly-time adversaries). You can find several pointers for example in the Cryptography in NC$^0$ paper.

Two specific examples include:

Integer multiplication (there are some subtleties for standard OWF, but if you only care about worst-case this is enough)

The Impagliazzo-Naor candidate based on subset-sum: $f(a_1,...,a_n,S) := (a_1,..., a_n, \sum_{i \in S} a_i \mod 2^n)$.

  • $\begingroup$ Thanks Emanuele. This answers the Logspace case. Just for completeness, Could you list some of those functions in your answer. $\endgroup$ Sep 1, 2013 at 20:06
  • $\begingroup$ I've added two examples. $\endgroup$
    – Manu
    Sep 1, 2013 at 23:14
  • $\begingroup$ Thanks a lot Emanuele. Is there an insight that explains the hardness of inverting those functions (by LOGSPACE algorithms)? $\endgroup$ Sep 2, 2013 at 13:35

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