code: private SecretKey clientKeyExchange(RSAClientKeyExchange mesg)
              throws IOException {
  
          if (debug != null && Debug.isOn("handshake")) {
              mesg.print(System.out);
          }
          return mesg.preMaster;
      }
docs: /*
  RSA key exchange is normally used.  The client encrypts a "pre-master
  secret" with the server's public key, from the Certificate (or else
  ServerKeyExchange) message that was sent to it by the server.  That's
  decrypted using the private key before we get here.