[wpixrptd] Designing the lifetime of a secret

How strong cryptographic parameters (e.g., key length) should one pick?  How long does the secret need to be kept?

Study history for secrets kept too short or too long.

Are there institutions who can take advantage of a secret divulged, say, 100 years later?  Romantically, one can imagine the descendants of one person inflicting revenge on the descendants of another.  But does this actually happen, and if so, under what conditions?

Are there financial things that one can do 100 years later?  Maybe vaguely something like replacing the Bitcoin root.

Weapons design is another area where secrets are often kept.  We have lost the secret of Greek Fire.