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I believe CMS (Cryptographic Message Syntax) might solve this problem. It allows sender to encrypt a packet and multiple recipients to open it. Basically it encrypts the contents with a random key and this key is encrypted for each recipient using various mechanisms.

Generate a symmetric key Encrypt symmetric key using public keys of all recipients, and broadcast (or provide on request from a client) Transmit all user messages encrypted under the symmetric key This means you only have one copy of the message (but you do have lots of copies of the encrypted symmetric key). Of course you need to solve the key ...