JPEG image scrambling without expansion in bitstream size

Abstract

In this work, an algorithm is proposed to scramble an JPEG compressed image without causing bitstream size expansion. The causes of bitstream size expansion in the existing scrambling methods are first identified. Three recommendations on AC coefficients in the scrambled image are proposed to combat unauthorized viewing. As the first step of the scrambling algorithm, edges are identified directly in the frequency domain using solely AC coefficients without relying on any traditional methods. These edges then form a low resolution image of its original counterpart and the information is utilized to identify regions. The DC coefficients are encoded in region-basis to suppress bitstream size expansion while achieving scrambling effect. Experiments were carried out to verify the basic performance of the proposed scrambling method. For the parameter settings considered, most of the scrambled images are of smaller bitstream size than their original counter parts.

Beyond format-compliant encryption for JPEG image

“…However, Ong et al [14] proposed a sketch attack based on the DC coefficients and successfully draw the outline of the original image directly from the image encrypted by [11]. Recently, Minemura et al [13] improved Li et al's sketch attack [12] in terms of quality and proposed a format-compliant encryption method for JPEG. Although bitstream size expansion is suppressed with occasional gain in compression, the proposed method mainly focuses on the DC coefficients while the potential of AC coefficients in achieving other objectives is left unexplored.…”

“…Note that, for the rest of discussion and experiments, we focus on the luminance signal (i.e., Y channel), but the descriptions are applicable to chrominance signals (i.e., Cb and Cr channels). Section 3.1 briefly reviews the format-compliant encryption technique [13] adopted in the proposed BFE to manipulate the DC coefficients. Section 3.2 processes the AC coefficients to reduce the bitstream size while Section 3.3 encrypts the AC coefficients.…”

“…To handle the DC coefficients, the perception encryption technique proposed by Minemura et al [13] is adopted. The basic idea is to rearrange the DC components based on the regions induced by the AC coefficients to achieve format-compliant encryption.…”

“…While many format-compliant encryption methods are designed to handle compressed image, most of them either are not robust to sketch attacks [12][13][14] or suffer from bitstream size expansion [12] or both [10]. In addition, the conventional format-compliant encryption methods for image are generally designed to serve only a single purpose, i.e., encryption.…”

“…In addition to format-compliant encryption, data embedding feature is offered to encode external information, which can be utilized for various purposes. First, the DC coefficients are encoded based on the regions induced by the textural information carried by the AC coefficients [13]. Second, AC coefficients are scanned in eight different orders where the order producing the smallest bitstream size is selected.…”