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Source: https://writers.stackexchange.com/q/33342
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

I'm a bit stuck on what should be a straightforward plot issue but not sure what is actually realistic.

The character is going away into _tera incognita_ (effectively exile until a deadly enemy has surely died, who will stop at nothing to hurt their family if it'll hurt the character). They may come back in 30 - 40 years time, if still alive. Or they may die alone; nobody at home will be aware either way. Essentially they will drop off the map and that's the end of all contact or knowledge of them unless by some chance they decide to return home in old age.

The character wishes to ensure a letter is delivered to their first grandchild (I'm handwaving away "what if their daughter doesn't have any children?") in about 40 years time. My question is, what would a realistic way be, to ensure this is done?

The story is set in the UK, in today's world. There's no magic or non-realistic elements, and the story gains a lot of its force from the care taken to research and portray matters accurately. It's also in a very "everyday" style, so it would be fine if they instruct some professional or put a document in a bank for storage. Money to pay for the service isn't a problem. What I'm stuck on is how they would actually do it, if someone really wanted to.

- They can't leave it with family or friends. Family / friends might let slip, or it may be seen by someone or just be misplaced or forgotten over the years (if the person who knows about it has an accident). 40 years is a long time for someone to remember they have a thing to do.
- If they wanted to leave it with a professional, the same applies - how would one be sure that a law firm would actually remember and not send the file out for shredding in 15 years? Do law firms ever have things on file to be done in decades time, in real life, or do they realistically require a person to contact them and remind/instruct them at the point in time when it needs doing? 
- Would it work to set up a trust with the law firm as trustee and an outside auditor, and an extra trust deed requirement that the auditor checks the trustees are acting as directed? So that the law firm would have to administer it and keep it "active", even if for 39.9 years all it did was pay legal fees? But if the law firm didn't, who would the auditors tell, or what could the character write into the instructions to deal with it?
- Would one pay a law firm to check the file annually, or some such, just to ensure it was never archived or forgotten?
- If not, what would someone do in today's society, if it really mattered?

The item - actually 2 items - to be delivered are the two halves of an encryption key + encrypted message pair. The character leaves the message to be delivered this way, by 2 routes that don't know of the other's existence, because then the 2 people/firms delivering them cannot read the message, since one only has encrypted text and the other only has an encryption key. But when the grandchild obtains both, they will be able to read the message, which directs them to any of 3 safety deposit boxes at different banks, all containing an identical envelope, and also tells them the code required to authenticate as the deposit box's owner. The latter part is easy since banks can be paid in advance (they hold mortgage documents for decades and at least one of 3 major UK banks will probably still be round, even if taken over, in decades time). But ensuring the messages will be delivered is less obvious.

Any help much appreciated, hopefully this is "in topic" to ask!

Original score: 3