The possibility of adding several watermarks to the same image would enable many interesting applications such as multimedia document tracing, data usage monitoring, multiple property management. In this paper, we present a novel watermarking scheme which allows to insert and reliably detect multiple watermarks sequentially embedded into a digital image. The proposed method, based on elementary linear algebra, is asymmetric, secure under projection attack and robust against distortion due to basic operations such as storage, transmission, format conversion, etc