The TLS session, and the 'secret inputs' to the TLS handshake, will always give proof that the received message originated from a TLS session from someone who h

The TLS session, and the 'secret inputs' to the TLS handshake, will always give proof that the received message originated from a TLS session from someone who held the private key to the used certificate. Or from someone who received a session key from that private key holder. If you want to fix this, you need the authenticity of the ciphertext to be proven to the recipient without the recipient being able to transfer it. An interactive zero knowledge proof could maybe do that.