A digital object identifier refers to diverse technologies associated with assigning an identifier to a digital resource and managing the identification system. One type of implementation of a digital object identifier developed by the Korean Government is termed the Universal Content Identifier (UCI) system. It circulates and utilizes identifiable resources efficiently by connecting various online and offline identifying schemes. UCI tags can contain not only identifiers but also abundant additional information regarding contents. So, researchers and practitioners have shown great interest in methods that utilize the two-dimensional barcode (2D barcode) to attach UCI tags to digital contents. However, attaching a 2D barcode directly to a digital content raises two problems. First, quality of the content may deteriorate due to the insertion of the barcode; second, a malicious user can invalidate the identifying tag, simply by removing the tag from the original content. We believe that these concerns can be mitigated by inserting an invisible digital tag containing information about an identifier and digital copyrights into the entire area of the digital content. In this study, to protect copyrights of digital contents securely without quality degradation, we attempt to discover a sequence of process for generating a 2D barcode from a UCI tag and watermarking the barcode into a digital content. Such a UCI system can be widely applied to areas such as e-learning, distribution tracking, transaction certification, and reference linking services when the system is equipped with 2D barcode technology and secure watermarking algorithms. The latter part of this paper analyzes intensive experiments conducted to evaluate the robustness of traditional digital watermarking algorithms against external attacks.