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Steganography
Steganography means hiding secret information in publicly accessible data; for example it is possible to hide text messages in a digital photograph by slightly modifying the colors of the image pixels -- that photo then looks just like an innocent picture while in fact bearing an extra information for those who can read it. Steganography differs from encryption by trying to avoid even suspicion of secret communication.
There are many uses of steganography, for example in secret communication, bypassing censorship or secretly tracking a piece of digital media with an invisible watermark (game companies have used steganography to identify which game clients were used to leak pre-release footage of their games). Cicada 3301 has famously used steganography in its puzzles.
Steganography may need to take into account the possibility of the data being slightly modified, for example pictures exchanged on the Internet lose their quality due to repeating compressions, cropping and format conversions. Robust methods may be used to preserve the embedded information even in these cases.
Some notable methods and practices of steganography include:
- Embedding in text, e.g. making intentional typos in certain places, using extra white or zero-width characters, modifying formatting and case or using Unicode homoglyphs can all carry information.
- Embedding in images. One of the simplest methods is storing data in least significant bits of pixel values (which won't be noticeable by human eyes). Advanced methods may e.g. modify statistical properties of the image such as its color histogram.
- Embedding in sound, video, vector graphics and all other kinds of media is possible.
- All kinds of data can be embedded given enough storage capacity of given bearing medium (e.g. it is possible to store an image in text, sound in another sound etc.).
- Information that's present but normally random or unimportant can be used for embedding, e.g. the specific order of items in a list (its permutation) can bear information as well as length of time delays in timed data, amount of noise in data etc.