[gillesjacobs]

A smart contract blockchain will allow you to deploy time and person-bound programs (though using exact-future-date as on-chain conditions is non trivial). An example is time-locked wallets. [1] Messages, files, money: you can put it all in there (big files won't be cheap).

My best bet is on Ethereum still being around in 20 years as it so entrenched.it'll be there for you. As long as people want to pay for using a blockchain to validators.

Non-technology, I would use a notary.

1. https://www.toptal.com/ethereum-smart-contract/time-locked-w...

[pcthrowaway]

How would you send a message on ethereum that would be encrypted before hitting the chain, and could be decrypted after some amount of time, without being decryptable before that? Any straightforward approach seems like it'd just rely on an off-chain decryption key, which doesn't really make it any better than putting letters in envelopes with the dates at which they should be opened.

Sarcophagus looks like the closest thing available right now, but it's still unproven enough that I wouldn't really count on it working 20 years from now: https://sarcophagus.io/

Time-locked wallets solve the potential issue of transferring value at some point after death, but can't be used to transfer messages.. not ones which aren't readable at the time you send them anyway