I have two ideas that may work, but their are 3 down sides.
1: You have to be smart. By smart I mean able to do plan out the Voyager missions by your self levels of smart.
2: You have to be rich enough to launch a payload into interplanetary space. The use of a company to send the payload for you is allowed as long as they do not see you put the message in the safe.
3: The time from sending message to receiving message must be at least
3 years, if the planets are in alignment (metaphorically and literally). That is just a rough estimate, but you get my point.
Build a satellite with the following in it:
1)Solvent (a fluid)
2)Solute (a solid)
3)An accurate accelerometer
4)An external thermometer
5)Plenty of fuel
7)An accurate clock
8)Strong 'hollow' safe
9)Really small piece of an explosive
10)Whatever something that measures magnetic fields is called
Physically write or encode your message on the solute (https://xkcd.com/1353/
). Put it in the safe. The walls of the safe have 5 layers. From the outside going in, they are; protective material, glass, solvent, glass, lead. The glass is a hollow taurus (donut) filled with the solvent.The glass in under tension like a Prince Rupert's Drop. This means it is strong, and more importantly, a break in any part of it causes the whole thing to shatter violently. The lead is very thin foil, just enough to stop radiation. If the glass shatters, it should be able to rip through the lead with very little resistance. As an alternative, you could make the glass with lead in it. This system ensures that anyone attempting to force through a wall of the safe will break the glass, causing the released solvent to dissolve the solute. With the solute no longer in a physical form, the message has been lost. It is recommended that you pick a solute-solvent combination react quickly and that a small sample of the saturated solvent can hold a lot of solute. Write a message explaining all of this on the outside of the safe, ensuring anyone who values your message will not force their way through the safe. To put it simply, the receiver must open the safe using the lock, as you intended. Also, the solute should react violently with radiation.* I will explain why later.
Now for the time factor. Build the satellite I described above and put the safe in it. Launch the satellite so that it reaches the outer planets. It may require gravitational assists (GA) from the inner planets. This is the most dangerous part because while it is in the inner planets, an eavesdropper (Eve) can launch a satellite to intercept it. I will explain later how to fix this problem. While near the outer planets, two general paths can be taken.
1) Send the satellite on a course that will give it a GA from Jupiter toward Saturn. Saturn uses a GA to send it to Jupiter or Uranus. The satellite will be passed back and in this way between the outer gas giants. This will ensure that intersecting it with another satellite will be incredible hard. It would make Rosetta look like child's play. After t time has passed, the satellite is sent back to Earth. It then lands on the Moon, or it enters orbit around the either the Earth, the Moon, the Sun, or a Lagrange point. The point is that it is now easily accessible (relatively speaking) to the people on Earth.
2) The second method is that you put the satellite in an orbit around the Sun and as far out as the Gas Giants. The satellite orbits from far way for a while. When the atomic clock hits a certain number, it begins its trip back.
No matter what we do, there is always a chance that Eve could send a satellite that intercepts your's. This is very easy to prove. If our satellite got there, then Eve's satellite can get there too. That is what the sensory equipment is for. If another satellite manages to intercept our own and started moving toward earth, then the accelerometer will detect the change in acceleration. If the other satellite is accelerating so slowly that our accelerometer cannot detect it (such as an ion engine), then the emissions from the engine will be detected by the thermometer or the magnetic detector thingy-a-bobber. If the satellite has a mechanical arm and that opens the mechanical lock, the solute will be exposed to radiation and is altered enough that the message is lost. If a space station, with people on it, intercept the satellite and try to open it in the station, (where there is little radiation) then the thermometer will detect the sudden rise in temperature. When any of the equipment detects a change, it will send the data to the computer. The computer will then check the clock and its records to see if this data is expected. Then the computer will detonate the explosive. Just to be clear, the explosive is inside the safe and is a shape charge designed to break the glass taurus. Even under ~0g some of the solvent will float toward and touch the solute, destroying the message. If this is not enough, you could always set the lock mechanism such that when it is opened it tells the computer.
The best way to test how secure your system is by measuring how much resources Eve would have to use to get it. The unit on this measurement would be Dollar-Time.
*I have to give credit for this to The da Vincei Code
. I read the book, so I do not know if this is in the movie. There is basically a safe that works like a bicycle lock with some interesting mechanisms in it. It contains a glass bottle of vinegar that has a 'paper' wrapped around it. The 'paper' is made out of something that dissolves in vinegar. As, you can guess, the message is written on the 'paper'. If the rings on the safe are lined up to spell the password, the safe can be pulled apart and the message is yours. However, if you try to pull apart the safe with the wrong password, pistons in the safe will move inward until they smash the bottle. The vinegar is released, dissolving the 'paper' and destroying the message.