Recovering DC Coefficients in Block-Based DCT

Abstract

It is a common approach for JPEG and MPEG encryption systems to provide higher protection for dc coefficients and less protection for ac coefficients. Some authors have employed a cryptographic encryption algorithm for the dc coefficients and left the ac coefficients to techniques based on random permutation lists which are known to be weak against known-plaintext and chosen-ciphertext attacks. In this paper we show that in block-based DCT, it is possible to recover dc coefficients from ac coefficients with reasonable image quality and show the insecurity of image encryption methods which rely on the encryption of dc values using a cryptoalgorithm. The method proposed in this paper combines dc recovery from ac coefficients and the fact that ac coefficients can be recovered using a chosen ciphertext attack. We demonstrate that a method proposed by Tang to encrypt and decrypt MPEG video can be completely broken.

On the Performance of Secret Entropy Coding: A Perspective Beyond Security

“…Unfortunately, if too many syntax elements containing perceptual information are left unencrypted, the security might be compromised. A lot of research [1,8,23,24,26,31,37,39,41] has shown that some perceptual information can be recovered from various kinds of unencrypted syntax elements. This problem is due to the following fact: most multimedia coding standards are designed in such a way that many syntax elements can be decoded independently without decoding other syntax elements.…”

2012 Proceedings of the Fourteenth Workshop on Algorithm Engineering and Experiments (ALENEX)

“…that missing DC coefficients cannot be effectively recovered, before Uehara et al reported in [22] that one can approximately recover missing DC coefficients by exploiting the fact that the difference of neighboring pixel values in natural images observes a Laplace distribution with zero mean and small variance [18]. Their method generally works well and is computationally efficient, but the perceptual quality of the recovered image is not always good enough.…”

Fast and Secure Real-Time Video Encryption

“…We observed that around 94.04% of the ACs lie in the range of 0 to 20. This is due to the fact that lower frequency AC coefficients carry less energy [14]. Figure 3 shows the percentage occupied by the quadruples, whose AC coefficient ranges is in 0-20 (orange) and 20-255 (dark-blue).…”

“…The encryption of the DC value plays a key role in the visibility of the video block. Some encryption algorithms only encrypt the DC coefficient while simply permutating the ACs [14]. In order to enhance the security, we apply CBC mode for encrypting the DC coefficients rather than the general ECB mode which is applied for the AC coefficients.…”