Encrypting only AC coefficient signs considered harmful

We show that the selective encryption of AC coefficient signs in DCT-based video formats is not suitable for use cases which require confidentiality, but can be used for some other application scenarios. By proposing a new assessment based on the method by Wu et al. we analyze the range of formats from the older JPEG standard to the more recent H.265/HEVC standard. We conclude that commonly used measures like exemplary PSNR values or key space calculations for theoretical brute force attacks are not sufficient to draw conclusions on the security of selective AC coefficient sign encryption.