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PDF attachments are very useful for lossless steganography. Image-based techniques get lost in recompression (e.g. Save To Camera Roll on an iPhone, or sending

PDF attachments are very useful for lossless steganography. Image-based techniques get lost in recompression (e.g. Save To Camera Roll on an iPhone, or sending via Facebook message). PDF attachments don't get lost in that way. Want to include the CSV raw data with your report? Just add it as a PDF attachment. Want to hide a game with your homework? Add it as a PDF attachment. Chrome and Preview on Mac doesn't show that it exists, but Firefox can be used to extract the file. It's not going to shock anyone to have a 5 MB file as a PDF, but there's a lot you can hide in there (MP3s, games, HTML files including more JavaScript, whatever else). On the surface, everyone thinks it's just another PDF. But the real data is hiding in plain sight.

maxerickson 6y ago
Using a standard feature to embed data isn't steganography. It's probably underutilized overall, but there's nothing hidden about it when most viewers show the data.
- peterburkimsher 6y ago
  Fair enough that it isn't steganography, though it can be used for similar applications. Do "Most viewers" show it? Google Chrome doesn't, nor does Preview on a Mac. There is no easy way to add attachments to a PDF, except Adobe Acrobat Pro or iTextPDF. Firefox and Adobe Reader can read the attachments, but it's "hidden" to some degree, inside slide-out side menus. Certainly enough to avoid a casual glance.
  - mkl 6y ago
    Okular alerts you to the presence of attachments, IIRC. I wouldn't say "most" though.
userbinator 6y ago
It's not going to shock anyone to have a 5 MB file as a PDF If I see a PDF containing a page or so of text and it turns out to be several MB, I would become a little suspicious. But you're right that most people are not aware of the general size of things.
- Zuider 6y ago
  With PDF, it would not be hard to obscure the presence of only a minimum of legitimate content using many page-breaks to give the illusion of a long document, and filling those pages with space-hogs such as large headers, tables, and algorithmically generated graphics. The sparsity of genuine content would not be too surprising given that PDF was originally intended for printing rather than reading.
- BlueTemplar 6y ago
  Just add a few pictures that double the weight, and nobody is going to notice.

maxerickson 6y ago

Using a standard feature to embed data isn't steganography. It's probably underutilized overall, but there's nothing hidden about it when most viewers show the data.

peterburkimsher 6y ago
Fair enough that it isn't steganography, though it can be used for similar applications. Do "Most viewers" show it? Google Chrome doesn't, nor does Preview on a Mac. There is no easy way to add attachments to a PDF, except Adobe Acrobat Pro or iTextPDF. Firefox and Adobe Reader can read the attachments, but it's "hidden" to some degree, inside slide-out side menus. Certainly enough to avoid a casual glance.
- mkl 6y ago
  Okular alerts you to the presence of attachments, IIRC. I wouldn't say "most" though.

userbinator 6y ago

It's not going to shock anyone to have a 5 MB file as a PDF If I see a PDF containing a page or so of text and it turns out to be several MB, I would become a little suspicious. But you're right that most people are not aware of the general size of things.

Zuider 6y ago
With PDF, it would not be hard to obscure the presence of only a minimum of legitimate content using many page-breaks to give the illusion of a long document, and filling those pages with space-hogs such as large headers, tables, and algorithmically generated graphics. The sparsity of genuine content would not be too surprising given that PDF was originally intended for printing rather than reading.
BlueTemplar 6y ago
Just add a few pictures that double the weight, and nobody is going to notice.