def compute_master_secret(self, pre_master_secret,
                              client_random, server_random):
        """
        Return the 48-byte master_secret, computed from pre_master_secret,
        client_random and server_random. See RFC 5246, section 6.3.
        """
        seed = client_random + server_random
        if self.tls_version < 0x0300:
            return None
        elif self.tls_version == 0x0300:
            return self.prf(pre_master_secret, seed, 48)
        else:
            return self.prf(pre_master_secret, b"master secret", seed, 48)
Return the 48-byte master_secret, computed from pre_master_secret,
        client_random and server_random. See RFC 5246, section 6.3.