I am working towards building a format-compliant encryption system for pictures. The aim of it is to be able to obscure specific areas of a picture (i.e. faces, car license numbers...) while keeping the rest unaltered. The aim would be, for instance, being able to publish a picture on any given social network, making the "sensitive" regions visible only to a limited number of people, probably a subset of your friends, whom have been granted permission.
I had some (arguable) ideas regarding pixel permutations and BMP, but I have stumbled face-first on the fact that most social networks use exclusively JPEG, and any other format is compressed, essentially ruining the scheme.
So now I am stuck trying to find a new encryption scheme. Ideally, it should have the following properties:
- Format-compliant: the image should be still a valid JPEG, openable and viewable.
- Unobtrusive: as much as possible, the encrypted image should not have a chunk of randomly colored pixels on the encrypted areas, but keep somewhat close to what it "should" look like - something like blurring or pixelation, but reversible. The rest of the image should be unaltered.
- Secure: aside from the crypto perspective, from a perceptual point of view the image should be obscured enough so as to make, e.g. a face unrecognisable without decoding.
2 and 3 are somewhat opposite, so my aim is finding some acceptable middle ground between them.
I have been trying to work with Droogenbroeck and Benedett's "Techniques for a selective encryption of uncompressed and compressed images", which proposes, for every 8x8 square, keeping the DC (aka most significant) coefficient of the DCT, and encoding the rest. However, it works pretty badly for mid-large images: since encryption is done on small squares, and the DC coefficient -which holds most of the energy- is left unaltered, the larger the image is, the "smaller" those 8x8 become in proportion, eventually becoming useless for images over about 500x500. Other than that, most papers I have found use JPEG2000 - which apparently would make everything much easier, but basically isn't supported by anything, so it doesn't seem a reasonable solution for now.
I know this is a bit on the open-ended-questions side, so specifically, my question is: Is there previous work, or any already discussed solution, for this problem or some other similar one? Do you have any ideas about how to approach this?
Thank you for your time, and please let me know if there is I can clarify - ideally I should link pictures of what I mean, but I don't think I'm allowed below a certain rep threshold :(
EDIT: As fgrieu noted, this has an added problem [please see below]. For example, Facebook apparently decompresses and recompresses the image on uploading, which means an additional, unavoidable quality loss. That means "fine-grained" tricks with coefficients (especially the least significant ones) would not work. Also thanks to Ilmari Karonen for linking the paper!
RE-EDIT: The problem pointed by fgrieu is avoidable in other social networks - for instance, Google+ does not recompress the image, so no problems there.