I'm attempting to send a message to a group of recipients, through an untrusted intermediary.
Each of the recipients will have been invited to the group by someone who already has a trusted public key from them.
The best solution I've come up with so far is that a shared secret is created by the first member of the group, and that is sent encrypted to each of the future recipients by whoever invited them. All messages addressed to the group will be encrypted with the shared secret.
Are there any major flaws with this approach? Is there a better, more well known approach?