def test_mismatchedHostKey(self):
        """
        If the SSH public key presented by the SSH server does not match the
        previously remembered key, as reported by the L{KnownHostsFile}
        instance use to construct the endpoint, for that server, the
        L{Deferred} returned by L{SSHCommandClientEndpoint.connect} fires with
        a L{Failure} wrapping L{HostKeyChanged}.
        """
        firstKey = Key.fromString(privateRSA_openssh).public()
        knownHosts = KnownHostsFile(FilePath(self.mktemp()))
        knownHosts.addHostKey(
            networkString(self.serverAddress.host), firstKey)
        # Add a different RSA key with the same hostname
        differentKey = Key.fromString(privateRSA_openssh_encrypted_aes,
                                      passphrase=b'testxp').public()
        knownHosts.addHostKey(self.hostname, differentKey)

        # The UI may answer true to any questions asked of it; they should
        # make no difference, since a *mismatched* key is not even optionally
        # allowed to complete a connection.
        ui = FixedResponseUI(True)

        endpoint = SSHCommandClientEndpoint.newConnection(
            self.reactor, b"/bin/ls -l", b"dummy user",
            self.hostname, self.port, password=b"dummy password",
            knownHosts=knownHosts, ui=ui)

        factory = Factory()
        factory.protocol = Protocol
        connected = endpoint.connect(factory)

        server, client, pump = self.connectedServerAndClient(
            self.factory, self.reactor.tcpClients[0][2])

        f = self.failureResultOf(connected)
        f.trap(HostKeyChanged)

        If the SSH public key presented by the SSH server does not match the
        previously remembered key, as reported by the L{KnownHostsFile}
        instance use to construct the endpoint, for that server, the
        L{Deferred} returned by L{SSHCommandClientEndpoint.connect} fires with
        a L{Failure} wrapping L{HostKeyChanged}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_endpoints.py