[Valgrim]

One could build a service just around the fact that the fastest theorical time it takes to transmit information around earth is around 130ms. This means that the absolute maximum number of layers that could be used in a day is well below 1 million. Scale that over a few billion layers or more, and reveal a new key every roundtrip, and you would've got yourself a much simpler timelock encryption.

The problem that arise is (and which is solved by the spacecraft to Neptune) is that with any earth based system, someone could secretly move copies of both ends closer together, secretly, and decrypt the lock faster than expected. Putting a spacecraft on a trajectory with no realistic chance of ever coming back makes this possibility impossible (as long as the layers of keys are encrypted only when the spacecraft arrived at its destination. Even if the delay between earth and neptune vary wildly, it is predictable, and any local system could piggyback a larger scale system like this for safety