In this paper, a secure card authentication system based on digital watermarking is presented. A sequence of identification characters, e.g. owner's ID number is inserted into the photograph, thus a linkage between the photograph and its owner can be established. The print-and-scan process is successfully overcome by our transform coefficient polarity based watermarking algorithm. The retrieval algorithm is capable of verifying whether the inspected card is authorized. We apply our algorithm for 105 photographs and the success rate achieved is better than 96%.