Somebody finds an effective NP-complete algorithm.

Then he breaks quantum ciphers by reversing time (time can be made backward by quantum symmetries, so the difference between forward and backward is that in backward direction the laws of physics are a cipher) and acting backward in time bringing quantum states to a state desirable for him.

Viable or no?

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    $\begingroup$ Welcome Porton. I'm concerned that uncertainty (indeterminacy) works the same in both directions, so you'd be working with probability functions rather than ciphers. $\endgroup$ Mar 26, 2022 at 22:32
  • $\begingroup$ @EveninginGethsemane A physicist in Internet convinced me (it's a too long lecture to repeat now) that we live in weakly interacting many-worlds quantum mechanics. The only ultimate reality of our universe is wave function, measurements are just approximations of the wave function. Apparently, in backward direction there is no uncertainty, because worlds merge rather than split. $\endgroup$
    – porton
    Mar 26, 2022 at 22:35
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    $\begingroup$ It doesn't matter what you believe. It matters what you choose for your world. For instance as an atheist (who strongly believes there are no gods) I can still create worlds with gods in them. We can't build your world for you or tell you whether some technobabble will or will not solve NP problems in polynomial time. $\endgroup$
    – sphennings
    Mar 27, 2022 at 3:10
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    $\begingroup$ This question needs some meat on its bones. What's a "quantum cipher"? Secure quantum codes rely on a single quantum state being measured at two distant locations. The sort of prime number ciphers used by PGP etc aren't quantum, but can be broken with the P=NP algorithm. And what does any of it have to do with the past? There might be a decent idea here but I'm not getting it. $\endgroup$ Mar 27, 2022 at 13:43
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    $\begingroup$ Unfortunately the question is gobbledygook. It reads like a bunch of sci-fi buzzwords and I cannot tell what you have in your head. $\endgroup$
    – Daron
    Mar 28, 2022 at 13:51

1 Answer 1


The usefulness of quantum communication is in key exchange, but also in the way that it makes any attempt to eavesdrop on a communication channel obvious, because the eavesdropped will collapse the quantum states being transmitted. It doesn't matter if the observer can interpret the contents of the channel (and P=NP won't help them there) because their interference cannot be hidden. note though that this can constitute a denial of service making it impossible to send information across the channel, but that's not the same as knowing what is being sent

P=NP would render the key-exchange process pointless for all currently known symmetric ciphers (because 3SAT is NP-complete) and probably all our current asymmetric ciphers too (though it isn't clear at this point if eg. the discrete logarithm problem is in NP), but one can still use a quantum communication channel to securely distribute a one-time pad, safe in the knowledge that it cannot have been observed by a third party without your noticing (and if it was observed, throw it away and start again). P=NP doesn't crack one-time pads, so secure communication is still possible.

All that remains is to find a suitable encryption algorithm in a higher complexity class. We don't have one yet, but I suspect it is only a matter of time.

  • $\begingroup$ It seems you didn't understand me: I propose breaking quantum ciphers by acting backward in time using a solution of P=NP. $\endgroup$
    – porton
    Mar 28, 2022 at 8:35
  • $\begingroup$ @porton yes, which sounds like garbage to me. It doesn't matter whether you're right or not though, because by interfering with a quantum communications channel your actions will be revealed to the communicating parties, who will repeat key exchange until interference goes away, possibly using some other communication channel. All you can do is a denial of service attack, and you can do that without magical handwavium. $\endgroup$ Mar 28, 2022 at 8:45
  • $\begingroup$ But I propose to break a quantum cipher before they even start to hide. $\endgroup$
    – porton
    Mar 29, 2022 at 11:19

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