code: def compute_master_secret(self, pre_master_secret,
                                client_random, server_random):
          
          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)
docs: Return the 48-byte master_secret, computed from pre_master_secret,
          client_random and server_random. See RFC 5246, section 6.3.