18.104.22.168.5 PUBLIC_KEY_REQUEST Message
The server MUST send this message to the client's RunspacePool if the server is trying to send secured data, and if the Session Key (section 22.214.171.124.7) is not available yet. See section 126.96.36.199.1 for more details.
The server sends this message to get the client's public key.
When this message is sent, the RunspacePool MUST be in an Opened state.