You may be familiar with image-based or audio-based steganography, the art of hiding messages or code inside of pictures, but that's not the only way to conceal secret communications. With zero-width characters, we can use text-based steganography to stash hidden information inside of plain text, and we can even figure out who's leaking documents online.
Image- and audio-based steganography has been covered several times on Null Byte, which involves changing the least significant digit of individual pixels on a photo or audio file. While plain text characters don't have a least significant digit that we can manipulate in the same fashion, we can still use Unicode to our advantage. Unicode is the standardized encoding format for text, specifically, UTF-8, that most web browsers use for text.
Because Unicode needs to support almost all written languages in the world, there are some counterintuitive characters such as zero-width non-joiners and zero-width spaces. For example, the zero-width non-joiner is used in languages such as Persian, where it's needed to display the correct typographic form of words.
In that image, notice how the line is no longer continuous? That's what is meant by a non-joiner. However, for our purposes, the most important part about these character types is that they're not ..
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