[Cryptography] Came up with a weird use case, got questions

[Arnold Reinhold]

On Sat, 5 Jan 2019 23:40 Joshua Marpet asked:

…
> Soooo, how about a system that automatically encrypts incoming emails? And
> then, some time later, it decrypts, long after anyone who wrote those
> letters is alive? Say 100 years. I'd write a letter to my mom which would
> be auto-decrypted 100 years later and given to the public. A la Post
> secret, and historical interest.
> 
> Any systems out there which will auto-decrypt, not based on a clock (which
> can be spoofed), but instead based on an event, a piece of information, a
> trusted info source? Something like the Long Now foundation's clock?

I remember a similar request being discussed many years ago, after some politician or the like was required to turn over his private diary to some legal proceeding. The idea was to come up with a public key that could be used to encrypt a diary, where the corresponding private key would only be released some time well in the future. One suggested approach was to generate keys for, say, ten-year intervals and secure the private keys with an m out of n secret sharing system. The key shards would then be entrusted to a dozen or so reputable agencies, such as university libraries, in different legal jurisdictions, who would agree to hold the keys in secret until the appointed times. There were also discussions of ceremonies to create the keys in a way that would be trusted. I remember the suggestion being made that any such ceremony protocol be reviewed be a good professional magician before being declared foolproof.

Another approach suggested, if I remember right, was to store the private keys in satellites placed in orbits that only return to earth infrequently.  With several  large satellite constellations in development or being proposed, all with much more computer capability than was common decades ago, adding a delayed key service might be quite feasible.

Arnold Reinhold