Researchers from Columbia University have created a technique called FontCode that can be used to embed hidden messages inside font glyphs (characters) The technique takes advantage of how computers work with fonts, which for them, are nothing more than mathematical equations used to draw lines and curves on a screen. An external observer can scan for these perturbations and decode them into indivudual letters based on a custom algorithm. The team hopes their FontCode system would be adopted in the real world. They say the system could replace QR code and watermarking systems.
Source: https://www.bleepingcomputer.com/news/technology/fontcode-technique-can-hide-secret-messages-inside-font-glyphs/