Comment by nonameiguess
2 months ago
More generally, you can use encoding formats that reserve uninterpreted byte sequences for future use to pass data that is only readable by receivers who know what you're doing, though note this not a cryptographically secure scheme and any sort of statistical analysis can reveal what you're doing.
The png spec, for instance, allows you to include as many metadata chunks as you wish, and these may be used to hold data that cannot be used by any mainstream png reader. We used this in the Navy to embed geolocation and sensor origin data that was readable by specialized viewers that only the Navy had, but if you opened the file in a browser or common image viewer, it would either ignore or discard the unknown chunks.
Lots of image formats store arbitrary metadata (and data data) either by design or by application-specific extensions. I remember seeing seismic and medical images that contained data for display in specialized applications and writing code to figure out if binary metadata was written in big-endian or little-endian byte order (the metadata often did not have the same endianness as the image data!) For example, TIFF files containing 3d scans as a sequence of slices, with binary metadata attached to each slice. If you opened it up in your system default image viewer, you'd only see the first slice, but a specialized viewer (which I did not have) would display it as a 3d model. Luckily (IIRC) the KDE file browser let you quickly flip through all the images in a directory using the keyboard, so I was able to dump all the layers into separate files and flip through them to see the 3d image.