Algorithmically speaking, Shamir's Secret Sharing seems of interest to your "combination of private keys" option. • https://en.wikipedia.org/wiki/Shamir%27s_Se

Algorithmically speaking, Shamir's Secret Sharing seems of interest to your "combination of private keys" option. • https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing • https://cs.jhu.edu/~sdoshi/crypto/papers/shamirturing.pdf https://cs.jhu.edu/~sdoshi/crypto/papers/shamirturing.pdf I agree with the sibling commentators though that it just isn't strong enough to withstand a threat model of "people motivated to get the master key to vast amounts of encrypted communication". Even if such a scheme couldn't be cracked algorithmically, it wouldn't hold up against the relentless social engineering efforts that would be thrown at it.