We investigate how to scale a free-space wavelength blocker to implement a spectral phase scrambling system to secure high data rate optical transmissions within a wavelength division multiplexing optical network. This technique belongs to the optical code-division multiple access family, and its implementation can be carried out using a free-space pulse shaping system based, for instance, on a wavelength blocker or spectral equalizer architectures. After a brief recall of the encryption principle, a model of the optical system is given, and we discuss the impact of phase distribution and the scrambling mask used to encrypt the data on penalty sources. We emphasize the importance of correctly choosing certain geometrical parameters, such as the beam waist in the spectral plane and the mask fill factor. The tolerance of this solution with respect to mask positioning errors is then investigated, and we finally discuss how such a solution could be implemented using existing devices, such as a wavelength blocker, for security applications without inducing additional system penalties.