We present an effective way to improve the security of a point-to-point terahertz wireless link on a physical layer supported by numerical calculations in the frame of Fourier optics. The improvement is based on original countermeasures which exploit three independent degrees of freedom of the carrier wave: its intensity and azimuthal and radial symmetry. When the transmission line is intercepted, the light beam is subject to changes in either of the three degrees of freedom. We propose a strategy to measure these changes and they are quantified by a single eavesdropping parameter that is shown to be correlated to the secrecy capacity of the transmission. Consequently, its excessive value serves as an indication of the beam interception. We consider the carrier wave in the form of Gaussian and vortex beams. Comparison between the two reveals that vortex beam ensures a even higher level of security.