A method for encryption of a digital watermark by intelligent halftoning, said method comprising: receiving image data that define at least a portion of a document to be printed in terms of at least three halftone images corresponding respectively to three printing colorants, each of said three halftone images comprising a plurality of halftone cells; modifying said image data by phase-shifting some of the halftone cells of at least one of said halftone images relative to the other halftone images to encode a watermark within said portion of said document, such that said at least one halftone image includes a phase-shifted region and a non-phase-shifted region, and wherein said watermark is a low-frequency watermark; using said modified image data to print said portion of said document on a substrate that will fluoresce when subjected to UV illumination, wherein said printed portion of said document includes a first printed pattern resulting from the phase-shifted region and a second printed pattern resulting from the non-phase-shifted region; wherein said first and second patterns of said printed portion of said document appear substantially similar when said portion of said document is viewed in visible light, and appear dissimilar when said portion of said document is viewed in UV light such that said watermark encoded in said modified image data is perceptible when said portion of said document is viewed in UV light and is hidden when said portion of said document is viewed in visible light, the method characterized in that said first and second printed patterns of said printed portion of said document comprise first and second rosette patterns, wherein preferably one of said first and second rosette patterns is clear-centered and the other of said rosette patterns is dot-centered. receiving image data that define at least a portion of a document to be printed in terms of at least three halftone images corresponding respectively to three printing colorants, each of said three halftone images comprising a plurality of halftone cells; modifying said image data by phase-shifting some of the halftone cells of at least one of said halftone images relative to the other halftone images to encode a watermark within said portion of said document, such that said at least one halftone image includes a phase-shifted region and a non-phase-shifted region, and wherein said watermark is a low-frequency watermark; using said modified image data to print said portion of said document on a substrate that will fluoresce when subjected to UV illumination, wherein said printed portion of said document includes a first printed pattern resulting from the phase-shifted region and a second printed pattern resulting from the non-phase-shifted region; wherein said first and second patterns of said printed portion of said document appear substantially similar when said portion of said document is viewed in visible light, and appear dissimilar when said portion of said document is viewed in UV light such that said watermark encoded in said modified image data is perceptible when said portion of said document is viewed in UV light and is hidden when said portion of said document is viewed in visible light, the method characterized in that said first and second printed patterns of said printed portion of said document comprise first and second rosette patterns, wherein preferably one of said first and second rosette patterns is clear-centered and the other of said rosette patterns is dot-centered.