@cwebber Good stuff!

Re: encryption "shelf life": would the URI scheme support multiple encryption?

Barring weaknesses in the actual ciphers (and the various other ways to undermine encryption), it's unlikely that data encrypted with modern ciphers at sufficient keysizes will ever be able to be decrypted without the key (Bremermann's limit, with the optimal brute-force post-quantum attack against symmetric ciphers being Grover's algorithm, which is mitigated by doubling the keysize).

So one option to mitigate the compromise of a cipher due to some sort of cryptanalytic attack is to use multiple ciphers, each with different keys.

Of course, if Alice is communicating an ephemeral symmetric key to Bob using a asymmetrically encrypted channel, the robustness of the symmetric algorithms won't matter much if attacker that can monitor network traffic between Alice or Bob may be able to decrypt that key exhcnage in the future. But that exchange could take place over a more trusted connection that is not available to the public, unlike the e.g. IPFS-stored encrypted messages themselves (though it may still be available to e.g. the NSA/GHCQ/etc). So there is still value in hardening the symmetrically encrypted message as much as Alice and Bob desire based on their threat model.