  In this paper, we study the secure degrees-of-freedom (SDoF) characterization
for the multiple-input multiple-output (MIMO) X channel with confidential
messages and delayed CSIT. In particular, we propose a transmission scheme,
which can be regarded as a generalization of the state-of-the-art scheme
without security and with delayed CSIT. The key of this generalization is
performing the security analysis, by which we derive the optimal duration of
the artificial noise transmission phase. As a result, we derive the sum-SDoF
lower bound. Furthermore, we reveal that if the number of receive antennas,
denoted by N, is fixed, the minimum number of transmit antennas achieving the
maximum of the lower bound is $\frac{7+\sqrt{33}}{8}N$.
