Many encryption problems are fast to solve if a key, factor, exponent or something similar is known. Other encryption methods don't have such keys. E.g. a hash value of another value. It's used because it is hard to determine which was the initial value. Computation time is needed for the solution. Is there a general name for this?

Especially I'm looking for methods which generate a chain. Starting at one value and computing the next value over and over again. Given two random values it should be hard to determine how many calculations are in between them. Is there a name for this?

In addition to the next value also the previous value should be computable in about the same time. Symmetric block cipher encryption is close to. But is there a more general name which is not limited to 'blocks'. Also different functions can be used for forward and backward direction.

Optimal case would be if besides next and previous element also selective offsets can be computed efficient but all other not. Is there a name for this?

  • $\begingroup$ key stretching? en.wikipedia.org/wiki/Key_stretching $\endgroup$ – user73280 Sep 28 '19 at 20:56
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    $\begingroup$ @dfhwze key stretching is more like an application which can use it. It can also done in other ways. I'm looking for a more a general name. But will have a look at key stretching methods. Maybe find some more interesting there. ty $\endgroup$ – J. Doe Sep 28 '19 at 23:17
  • $\begingroup$ So this is what you need! start a bounty! $\endgroup$ – kelalaka Oct 27 '19 at 22:20

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