-This work considers digital images as covers and foundation is
+This work considers digital images as covers and it is based on
spatial least significant-bit (LSB) replacement.
In this data hiding scheme a subset of all the LSB of the cover image is modified
-with a secret bit stream depending on to a key, the cover, and the message to embed.
+with a secret bit stream depending on a key, the cover, and the message to embed.
This well studied steganographic approach never decreases (resp. increases)
pixel with even value (resp. odd value) and may break structural symmetry.
These structural modification can be detected by statistical approaches
We argue that modifying edge pixels is an acceptable compromise.
Edges form the outline of an object: they are the boundary between overlapping objects or between an object
and the background. A small modification of pixel value in the stego image should not be harmful to the image quality:
-in cover image, edge pixels already break its continuity and thus already contains large variation with neighbouring
+in cover image, edge pixels already break its continuity and thus already contain large variation with neighbouring
pixels. In other words, minor changes in regular area are more dramatic than larger modifications in edge ones.
Our proposal is thus to embed message bits into edge shapes while preserving other smooth regions.
Contrarely, we argue that some images should not be taken as a cover because of the nature of their signal.
Consider for instance a uniformly black image: a very tiny modification of its pixels can be easily detectable.
The approach we propose is thus to provide a self adaptive algorithm with a high payload, which depends on the
-cover signal.
+cover signal.
+For some applications it might be interesting to be have a reversible procedure to compute the same edge detection pixel set for the cover and the stego image. For this, we propose to apply the edge detection algorithm not on all the bits of the image but only on all the bits without taking into consideration the LSB.
\JFC{Christophe : énoncer la problématique du besoin de crypto et de ``cryptographiquement sûr'', les algo déjà cassés....
