How printouts and photos can deanonymize you
Think printing a letter keeps you anonymous? Think again. Your printer is actually a snitch. Most color laser printers secretly add invisible yellow dots on every page, called the Machine Identification Code (MIC). This hidden code includes the printer’s serial number and the exact date and time the page was printed. You can’t see these dots, but the authorities can detect them using a special blue light. If they find the paper, they can track down the printer, and from there, they can find you.
In a similar way, phone cameras have noise signatures that are unique to the camera model and even to a specific device. In addition, every camera lens leaves a blur signature — like a hidden fingerprint in every photo (Learning Lens Blur Fields, Esther Y. H. Lin et al., TPAMI 2025). Not knowing about these small, hidden details could put you or your sources at risk. For instance, some media outlets have unknowingly endangered sources by publishing the original photos instead of digitizing them, leaving these hidden markers in the image.
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