[denton-scratch]

So encryption is a fixed-time operation, requiring a single round-trip to the spacecraft. There is another time-delay at decryption, quantized into round-trip times; the minimum decryption delay is one round-trip. For a moon of Neptune, a round-trip is about 9 hours(?).

If I want my secret exposed in 20 years, I will need to wrap it in 18,000 layers of encryption, and then start the decryption process immediately.

The duration of one decryption step depends on the distance to the spaceship; it would be difficult (but not impossible?) to rely on a spaceship whose distance is always changing. It needs to be somewhere faraway, and also to be somewhere that's always going to be roughly the same distance away. A moon of Neptune is a reasonable candidate.

[troymc]

The distance between Earth and Neptune oscillates with a period of about one year because Neptune moves around the sun much slower than the Earth does. The Earth-Neptune distance oscillates between about 29 AU and 31 AU.

I checked your calculations and I get similar numbers. I don't think that tens of thousands of layers of encryption is a problem: a modern computer can store that many private keys with no problems. In fact, it should probably store three copies of each one, or do something to account for random bit flips.

[denton-scratch]

> because Neptune moves around the sun much slower than the Earth does

I think of the "1AU either way" thing as being simply that sometimes the Earth is on the other side of the Sun from Neptune, so the Earth<->Neptune distance is 1AU greater; and sometimes it's on the same side, so 1AU closer.

I didn't mean to suggest that 18,000 layers was silly; for encryption it would take seconds or less. And for decryption, having multiple layers is vital, because that's what creates the delay.

It's awkward that you have to start decrypting immediately. The ideal would be a self-contained object like a USB key, that does just one thing: after n ticks, it exposes its secret. In principle, it could be based on a mechanical clock, but it's hard to envisage a tamper-proof mechanical clock.