  We consider an OFDM transmitter aided by an intelligent reflecting surface
(IRS) and propose a novel approach to enhance waveform security by employing
time modulation (TM) at the IRS side. By controlling the periodic TM pattern of
the IRS elements, the system is designed to preserve communication information
towards an authorized recipient and scramble the information towards all other
directions. We introduce two modes of TM pattern control: the linear mode, in
which we design common TM parameters for entire rows or columns of the IRS, and
the planar mode, where we design TM parameters for each individual IRS unit.
Due to the required fewer switches, the linear mode is easier to implement as
compared to the planar mode. However, the linear model results in a beampattern
that has sidelobes, over which the transmitted information is not sufficiently
scrambled. We show that the sidelobes of the linear mode can be suppressed by
exploiting the high diversity available in that mode.
