以小波轉換為基礎的數位浮水印之影像鑑定與篡改偵測

Abstract

網際網路的迅速擴張及數位科技的長足進步，增加了數位媒體的容易取得，造成媒體所有權擁有的合法性問題。浮水印技術是一種適合做為數位資料所有權識別的工具，它能夠在經由無法察覺的修改而加入隱藏的資訊。 本論文研究以小波轉換為基礎的半易碎型浮水印之影像鑑定與篡改偵測。為達到影像保護的目的，影像在小波分解及量化程序之後，嵌入一個安全的、以小波樹為基礎的二元影像簽章（wavelet tree based binary image signature, WTS）；除此之外，被選取的小波係數需經過係數微調的步驟，以增加浮水印的彈性。在影像鑑定過程中，不需要原始未加入浮水印的影像作對照；比較可能受篡改影像的WTS與經過確認的WTS，即能夠偵測影像是否受到未經許可的篡改。本論文提出的技術不只能夠定位出受到篡改的位置，而且有分辨非故意之偶然修改與惡意篡改的不同；這項技術能夠承受中等品質的JPEG壓縮，也能夠有效地偵測出影像的細微篡改。

The rapid expansion of the Internet and the recent advance of digital technologies have sharply increased the availability of digital media. In consequence, watermarking is developed as a suitable candidate for the ownership identification of digital data as it allows the invisible insertion of information with the imperceptible modification. This thesis is to investigate a wavelet based semi-fragile watermarking technique for image authentication and tampering detection. Image protection is achieved by the insertion of a secret wavelet tree based binary image signature (WTS) after wavelet decomposition followed by quantization procedure. In addition, tuning steps is performed for the selected watermarked coefficients in order to increase the elasticity of watermark. During the verification, the original unmarked image is not needed for comparison. The detection of unauthorized tampering within the image is performed by comparison with the possibly modified image’s WTS and the authentic one. The proposed technique not only localizes the tampered position, but also has the capability to distinguish incidental modification from the malicious tampering. It stays unaffected by medium JPEG quality compression and also effectively points out the small image modifications.