Methods include scrambling non-zero coefficients, permutating coefficients intra/inter sub-blocks. Encrypted images are noise-like under direct vision. But the amount of non-zero coefficients within each blocks change little, and which causes some information leakage.
· Stream cipher based image encryption:
Irrelevant design is vulnerable to cryptoanalysis.
Suppose S represents Pixel-level substitution, and P represents Pixel-level permutation. The combined structure SP is equivalent to PS, PP is to P, and SS is to S. If the structure of an algorithm is equivalent to S, P, or PS, then it can not resist chosen text attack, or then known plaintext attack.
We suggest introducing feedback mechanism into S, and use SPS structure. Note, if use PSP structure, there is at least one point are not feedbacked.
· A robust image encryption scheme:
Which satisfies: secure to normal attacks; secure to special attacks; encrypted image is robust to transmission noise; encrypted image is robust to normal processing; and encrypted image remain high compressibility.
