Perform the derivation of master_secret into a key_block of req_len
        requested length. See RFC 5246, section 6.3.

def derive_key_block(self, master_secret, server_random,
                         client_random, req_len):
        """
        Perform the derivation of master_secret into a key_block of req_len
        requested length. See RFC 5246, section 6.3.
        """
        seed = server_random + client_random
        if self.tls_version <= 0x0300:
            return self.prf(master_secret, seed, req_len)
        else:
            return self.prf(master_secret, b"key expansion", seed, req_len)