docs: 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)