[cnvogel]

Yes, b/w converting is not sufficient. Once printed, the yellow dots are hard to remove.

http://imgur.com/a/kLovh

And even when you mask them out so that they are no longer visible in the "all white" (paper) background, e.g. by messing with the white/black point of the image there's still the possibility that they could be recovered with correlation methods in grey areas where they aren't visible to the naked eye or just by increasing the contrast.

[thaumasiotes]

Why would there be grey in a thresholded image? The entire point of the transform is that it maps everything above a certain threshold to pure white and everything else to pure black.

They didn't say "convert to greyscale".

[cnvogel]

> They didn't say "convert to greyscale".

Very good point. But even then, assume that one page of a leaked document contains a large picture with areas around the thrshold value: With the agency being able to recreate a perfect replica of the initially scanned paper version, but without yellow dots, it might be possible to extract the (very few) bits necessary to boil it down to a single printer serial number by statistical methods.

[davidsong]

Hmm, okay, so we reduce to black and white, add some warp and noise and then reduce the size so that the text is only just readable.

...and they focus on adding fonts of multiple sizes so it can't be shrunk without losing information.

[thaumasiotes]

Reduce to black and white, and proofread for dots. If they're still there, they will be easy to see, since you only have two colors. You can white out an image that came out looking like a test pattern.

[imron]

2bpp BMP FTW!