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This question already has an answer here:

I'm wondering if there is any way to make crypto based on time, where the concept of time is linear in one-direction. Even a theoretical brainstorm.

Example 1:

A message which can only be opened/read after a certain time. Could be good for distributing things before a release date.

Example 2:

A message which cannot be opened be opened/read after a certain time. Could be good for HTTPS, or exchanging short messages between spies.

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marked as duplicate by Ella Rose, yyyyyyy, SEJPM, e-sushi Feb 22 '17 at 23:09

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Most of those answers are either wrong, include space borne lasers or simple ignore the fact that this is currently, practically and extremely effectively done all the time for the bitcoin system. None of them give a real world example of a proof of work based time delay. $\endgroup$ – Paul Uszak Feb 23 '17 at 3:28
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If the period for which the message can't be read it is < 2 weeks, then yes. This forum is littered with proof of work time based cryptography so I won't rehash them.

In reviewing the questions /answers here, no one seems to discuss that for short periods (<2 weeks say) it is entirely possible. The 2 week limit is a soft limit, but that's the difficulty index recalculation period for the bitcoin currency. bitcoin mining works and is proven to be an un ircumventable proof of work procedure. It must also be one of the most studied proofs of work in the world. It seems to make intuitive sense, that you might set a hardness parameter (much like in scrypt) based on current processing technology that will take up to 2 weeks to crack. You might set it for only 2 days, but there will be many applications where that would be adequate. Needless to say that the proof of work algorithm should not be able to be paralleled, perhaps something akin to the Rivest puzzle. Today you'd probably target the Intel Xeon E7 chip.

Just as bitcoin does, the 2 week limit eliminates the risk of mathematical /technological leaps in hash computation. We can debate the exact delay period, but that will have to include a trade off between risk of early decryption vs. potential damage. At a long stretch, this might be comparable to the standard definition of a hash security parameter, but measured in seconds, rather than bits.

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