[michelpp]

You've hit the nail right on the head with this question on how hard group encryption is, and we don't have all the answers yet as we are still working the use cases around it. We are hoping to reach a level of security that you mention in your SE question using something similar to the excellent accepted answer, distributed private key sharing among trusted participants.

The basis we are exploring is using an algorithm called Signcryption (https://github.com/jedisct1/libsodium-signcryption) that is already included in pgsodium. This doesn't solve any of the shared private key issues you mention above, but it is a useful foundation for distributing encrypted messages that separate out sender and receiver identifiers from their keys, a sort of lower level foundation on top of which distributed key sharing can occur.

I also think signcryption is a great foundation for a better token format than JWT or PASETO, as it covers all of their use cases without algorithm confusion attacks (despite PASETO's insistence on "Algorithm Lucidity") and supports more features such as third party verification and streaming shared key generation from any token without having to exchange the key, we hope to use these tokens so that end-to-end peers can exchange tokens, derive streaming shared keys, and then do direct point-to-point message exchange using libsodium crypto_secretstream API which supports key ratcheting for forward secrecy.

Would love to discuss more about your research with you and include it with attribution into our future work, send me an intro at [e-mail] when any other ideas or resources you'd like us to see!

[moralestapia]

Signcryption looks very interesting, indeed. I haven't taken a look at the space for a couple years but this renewed my interest.

I'll write you an email, for sure. Cheers!