本文指出存在於由 Zhang and Wang (2004)所提出之藏密法中之瑕疵，並提出 方法加以修正。Zhang and Wang 之方法，可消除由廣為人知之像素值差異 (pixel-value differencing, PVD)技術所產生之偽裝影像(stego-image)，其像素差值 直方圖(pixel difference histogram)中不尋常之階梯狀圖形，故而能抵抗使用直方 圖藏密分析(steganalysis)所進行之攻擊。然而，這個與用來將像素差值進行分類 之範圍(range)設定有關之瑕疵，由於其不當之寬度設定，在特定情況下，會導致 資料嵌入程序(data embedding process)失效。本文提出了能產生恰當之範圍寬度， 以確保資料嵌入程序能成功運作之方法，用來修正此一瑕疵。

This paper indicates a flaw in the steganographic method proposed by Zhang & Wang (2004) and proposes a method to repair it. Zhang & Wang’s method can eliminate the undesired abnormal steps in the pixel difference histograms of the stego-images generated by the well-known pixel-value-differencing (PVD) technique and thus is immune to histogram steganalysis. However, under certain conditions, the flaw, which is associated with the inappropriate setting of the widths of the ranges for classifying pixel differences, can lead to the failure of data embedding process. To repair the flaw, a method for generating appropriate range widths to guarantee the success of data embedding process is proposed.