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A NOVEL SEMI-PRIVATE WATERMARKING TECHNIQUE Peter H. W. Wong*, Oscar C. Au** Department of Electrical and Electronic Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong. Email: [email protected]*, [email protected]** Tel.: +852 2358-7053**

ABSTRACT A novel watermarking technique is proposed to embed a semi-watermark in digital images. Before embedding the watermark, a M-dimensional vector is extracted from the original data. Watermarking is achieved by modifying this vector to another particular vector according to the private keys and the watermark. A dual-key system is used to reduce the chance for the removal of watermark. The watermark can be detected without the original image. If the original image is available in the detection of watermark, the proposed technique is almost equivalent to the well known spread spectrum technique.

1. INTRODUCTION Spread spectrum technique [1] is widely adopted for image and video watermarking in the past few years [1-4]. Some of them [1, 2] are private watermarking schemes [4] which require the original data to detect the watermark. The robustness of these schemes is good against many signal processing such as JPEG compression and filtering. However, in some situations such as watermark detection in a DVD player, private schemes are infeasible. Semi-private watermarking schemes [2, 3] detect the watermark with the watermarking or key information but without the original data. The trade-off is often reduced robustness. A weakness of semi-private watermarking is higher chance of false alarm (declaring watermark presence when it is absent) compared to private schemes. In this paper, we propose a novel semi-private technique to embed a watermark in a watermark space. The proposed scheme is based on spread spectrum with some new features. In the embedding and detection stage, the watermark space is extracted and reordered to a M-dimensional vector from the original data and testing data respectively. This watermark space can be the subset of the spatio domain of an image or the subset of the frequency domain such as the DCT domain. With the use of a dual-key system, the embedded signal doesn’t need to be orthogonal to the original data. This property allows small images to embed the watermarks.

2. WATERMARK EMBEDDING Without loss of generality, the watermark space is assumed to be an M-dimensional vector space. In other words, the watermark is embedded into an M-dimensional host vector Y shown in Eqn. 1. The watermark W is assumed to be a bit sequence of length N as defined in Eqn. 2, where N