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Most applications that produce content (printout, image, video, Word document, Excel spreadsheet, web page etc.) pretty much anything that can be published, nowadays comes with metadata. Metadata is data about data.
Metadata are information summaries about data (in short, data about data), embedded in the document (image, web page HTML code). It is used for
The reason that we love standards is that there are so many of them! The same can be said about Metadata standards --> http://en.wikipedia.org/wiki/Metadata_standards. I do not know off-hand if there is a standard for binary-coded metadata, but there is no reason it cannot be done. For example, a JPEG2000 image is a binary file --- nevertheless, its metadata are in an XML format yet they are written in binary notation within the image file: http://www.jpeg.org/jpeg2000/metadata.html . Here is a huge long report on this: http://www.jpeg.org/metadata/15444-2.PDF (which I did not bother to read). :)
Another answer, from Patrick Maroney:
Steganography would be one method for achieving most of the objectives. One paper "A Secure Robust Image Steganographic Model" (2000) by Yeuan-Kuen Lee , Ling-hwei Chen
discusses approaches on improving the survivability/resilience of the encoded "covert channel". it can be found @http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.32.4617. (As always take appropriate measures to mitigate any risks from accessing potentially malicious content).
This has worked to watermark specific documents, logos, etc. (commonly used by modern adversaries to create attack packages using content from public facing websites). This can be a highly effective methodology to identify where adversary is acquiring content in their Reconaissance and Weaponization phases.
As to providing Google a way to securely index and search against covert metadata buried in a covert channel? "Tongue in Cheek" comment: What makes one think they (Google) aren't already capable of this and more?
That link to the paper is stale. The NCTU paper can be found on their web site: http://debut.cis.nctu.edu.tw/Publications/pdfs/C22.pdf (same caveat on taking appropriate meauses to mitigate any risks from accessing potentially malicious content).