In order for the butterfly effect to not disturb the results of any random or pseudorandom key, the key must be derived in such a way that the distance of the key generation event is greater than 1/2 the light-time of the key delivery delay, so that in no case can any information from the key holders reference reach the key source prior to key generation.
Other than that the key must be based in a purely deterministic algorithm to be immune to butterfly factors, which can always be brute forced by accelerating the civilization to fractional relativistic velocities or bringing it close to an extremely massive object. (Leaving the keygen at rest) Of course other ways to brute force such a key generator are probably more practical, but a relativistic attack is the steel man for the impossibility of reliability.
Stealth might be the best bet, an undersea bouy that pops up and broadcasts the key at the specified time or something like that. But then again, a fast moving civilisation would get the key “early” from their frame, even for astronomical events.
I posit that there is no absolutely reliable delay, but stealth is probably the best practical hedge.
It wouldn’t likely apply to 1980s tech, but a “fast moving” civilisation (or one capable of locally warping spacetime to a meaningful extent) could still receive the key in less than the allotted time.
But if we’re going to stay unrediculous, the nova -should- work , but might not if the many-worlds interpretation (which is misleadingly named) interpretation of QM turns out to be right.
The time traveler effectively brings the nova into the light-cone of the past world, altering the relationship in the MWI.
Other than that the key must be based in a purely deterministic algorithm to be immune to butterfly factors, which can always be brute forced by accelerating the civilization to fractional relativistic velocities or bringing it close to an extremely massive object. (Leaving the keygen at rest) Of course other ways to brute force such a key generator are probably more practical, but a relativistic attack is the steel man for the impossibility of reliability.
Stealth might be the best bet, an undersea bouy that pops up and broadcasts the key at the specified time or something like that. But then again, a fast moving civilisation would get the key “early” from their frame, even for astronomical events.
I posit that there is no absolutely reliable delay, but stealth is probably the best practical hedge.