Correspondence Recovering DC Coefficients in Block-Based DCT
Takeyuki Uehara, Reihaneh Safavi–Naini, Philip Ogunbona · 2006
It is a common approach for JPEG and MPEG encryption sys- tems 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 coeffi- cients with reasonable image quality and show the insecurity of image en- cryption methods which rely on the encryption of dc values using a cryp- toalgorithm. 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.