Supposing I have Alice that is the RA, Bob (who is a user) and Kate (who is also a user). For now I use RSA. Alice sends a message to Bob. Alice also sends a message to Kate. Bob and Kate exchange their messages as they took them from Alice (without decrypting them and encrypting them) using the public key of Alice. My question is how can Bob and Kate decrypt the messages using the public key of Alice? Is there any other protocol that should I use when I have 1 RA, and many users, and I want the users to decrypt the messages they got from the RA (without decrypting and encrypting in the mean time)? What I am trying to do here is Bob to get the message of Kate and do some checking, and also Kate get the message of Bob and do some checking.