Look inside an image for what should not be there STEGOSCAN
A picture file has places a picture does not need: bytes after the end marker, undefined chunks, comment fields, the low bit of every pixel. STEGOSCAN walks the container and shows you what is there.
Open the picture
PNG, JPEG, GIF, WebP or BMP. It is read in this tab; nothing is uploaded.
What stands out
What the file is made of
Anything after the end of the image
Text and comments carried in the container
Metadata
Printable runs in the raw bytes
The least significant bit of every pixel
Normally this looks like television static. Flat blocks, text, edges or a patch of different-looking noise are worth a look.
Each row is the whole picture. The sentence above also measures a growing prefix, because sequential embedding fills the beginning and stops, and a message occupying a tenth of a picture is a tenth signal to nine tenths noise when the whole histogram is taken at once.
What this test assumes, and when it is blind
The pairs-of-values test works because random bits written into the low bit drive the counts of each pair of values towards each other. That holds when the hidden payload looks random, which it does whenever it has been encrypted or compressed first, as any competent tool does, this site’s own included.
Write plain text straight into the low bits and the assumption fails. ASCII is not random, the pair counts do not balance, and this test reports an ordinary picture. It is, oddly, better at catching careful tools than careless ones, and the bit planes above are the thing that catches the careless ones: plain text in the low bits is often legible as shapes.
It is also blind to embedding that scatters across the picture from a key rather than filling it from the first pixel, and to anything hidden in a JPEG, where the payload goes into the compression coefficients and never touches a pixel value. And a payload under roughly a sixth of the capacity is too small to separate from chance at this resolution.
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