A method for watermarking an encrypted digital content stored in a content distribution network (CDN) method comprises the steps of receiving in the CDN, a non-watermarked encrypted content and marking metadata, said non-watermarked encrypted content being the result of an encryption of a non-watermarked clear content by a stream cipher process, said marking metadata indicating locations in the content suitable for a modification of said content; using a watermark formed of watermark values and corresponding to a unique identifier; and producing a watermarked encrypted content by combining in the CDN, with a combination function, the values forming the watermark with the non-watermarked encrypted content in locations indicated by the marking metadata.