A safe way to send critical information to a country with censorship / surveillance

[Togijak]

PDF Page Lock Pro 2.1.2.4 Portable was used to hide the critical information in a harmless PDF. In my test, I used this image as "critical information" hidden in the PDF

In Acrobat, I inserted a blank page after page 3 and inserted the picture there

Then the new page 4 was password-protected and hidden using PDF Page Lock Pro.

You can only make the hidden page visible again with PDF Page Lock Pro because it is not visible to other tools. If you compare the original and the version with the hidden information you will indeed find differences depending on the used comparison tool, but they are all only visual. Acrobat shows only the images as a difference when comparing documents.

However, these differences would not be noticeable because they are not in the possession of the original and even if one would reduce the minimum differences in comparison to different resolution, especially since in this test the PDF with the hidden page is smaller than the original.

Of course, to provide no indication of a link between the sender and the recipient, you should not send such a PDF by e-mail, but upload it to a page from where the recipient can download it.

If you want to check yourself if you can get the hidden information without the password, you can do that by downloading the two PDF's

https://www.upload.ee/files/9318400/DiffPDF-5.pdf.html

https://www.upload.ee/files/9318399/DiffPDF-5_with_hidden_page.pdf.html