A securing method implemented by a security system, the method comprising: - acquiring a first image characterized in at least N dimensions comprising a first and a second spatial dimension (DM1, DM2) and a third dimension, N being an integer greater than or equal to 3, said third dimension defining one of: o a spatial dimension other than the first and second spatial dimensions; o a colorimetric dimension in a color space; and ∘ a spectral dimension in a spectral space; the first image comprising a plurality of pixels defined by their respective position in the first and second dimensions and by at least two components (CP1-CP4) in said third dimension; - compressing the first image into a second grey-scale image comprising a plurality of pixels each having a grey scale representative of at least one component in said third dimension of a corresponding pixel of the first image, said compression comprising applying a transfer function to determine, from the components in said third dimension of each pixel of the first image, the grey scales of the corresponding pixels of the second image, the transfer function corresponding to a data matrix comprising, for each pixel of the first image, a respective value defining a transformation of the at least one component in said third dimension into a grey scale, the matrix comprising different values such that the at least one component in said third dimension of at least one pixel is transformed differently from the at least one component in said third dimension of at least one other pixel; and - forming the second grey-scale image on a support. - acquiring a first image characterized in at least N dimensions comprising a first and a second spatial dimension (DM1, DM2) and a third dimension, N being an integer greater than or equal to 3, said third dimension defining one of: o a spatial dimension other than the first and second spatial dimensions; o a colorimetric dimension in a color space; and ∘ a spectral dimension in a spectral space; the first image comprising a plurality of pixels defined by their respective position in the first and second dimensions and by at least two components (CP1-CP4) in said third dimension; o a spatial dimension other than the first and second spatial dimensions; o a colorimetric dimension in a color space; and ∘ a spectral dimension in a spectral space; - compressing the first image into a second grey-scale image comprising a plurality of pixels each having a grey scale representative of at least one component in said third dimension of a corresponding pixel of the first image, said compression comprising applying a transfer function to determine, from the components in said third dimension of each pixel of the first image, the grey scales of the corresponding pixels of the second image, the transfer function corresponding to a data matrix comprising, for each pixel of the first image, a respective value defining a transformation of the at least one component in said third dimension into a grey scale, the matrix comprising different values such that the at least one component in said third dimension of at least one pixel is transformed differently from the at least one component in said third dimension of at least one other pixel; and - forming the second grey-scale image on a support.