I don't see why I'd need Solitaire. There is probably going to be something much more seriously wrong than someone tampering with my diary if I end up in a future where I am unable to get my hands on any simple computer I can make run a contemporary crypto algorithm.

Basic scenario is, I don't completely trust the safety deposit box, so I want to put something in there that can't be easily changed and rebuilt (maybe the reanimators botch something bringing me back and then go mess with my personal effects trying to alter the evidence that could tip me off to something being wrong).

I could just use a standard digital signature with a huge private key which I would then destroy. But I'd need to store the public key somewhere outside the deposit box, or else the attackers could just re-sign the forgery and replace my public key with theirs in the box. I could commit the public key to memory, but might forget it with brain damage. I could also try to leave it in some public archives, since the key wouldn't contain any information I might want to keep private, like the deposit box would.