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Thursday, July 31, 2014

Can you match a photo to the discrete camera it was taken with, without metadata?

Over on Quora, a user asks the following question: "Are there enough digital and/or analog difference in individual cameras/houses/lenses that photos will have fingerprint of sorts?

When, or in what circumstance could a photograph be linked not only to a lens/house model or manufacturer but to one specific camera, distinguished from any other camera of the same model?" Essentially, can you match a photo to the discrete camera it was taken with, without metadata?

Amped Software's CEO answers the question.

The short answer

Yes, it is possible to match a photo to the discrete camera it was taken with (without metadata) and it is also pretty reliable. The technique is readily available in a few software products, one of those is Amped Authenticate, produced by Amped Software (disclaimer: I am the company CEO and Founder).

The basic idea

The basic idea is that every single device leaves a different “noise fingerprint” on each photo it produces. This component is called PRNU (Photo Response Non-Uniformity) and it has been widely studied in literature. It has been shown to be:

  • constant over time
  • constant over temperature
  • independent of other camera settings (exposure, focus, etc…)
  • fairly robust to recompression (up around JPEG quality 5-60%)
  • fairly robust to intensity and color adjustments (contrast, brightness…)
  • fairly robust to local modifications (i.e. if a part of the image has been tampered, the picture as a whole is still recognized as coming from a specific camera)

However, it does not work properly in these situations:

  • if the image has been cropped or has digital zoom, since it would take only a part of the sensor and not its whole area (this could be solved, but then it wouldn’t be robust to resize)
  • for very strong enhancements
  • for very dark or very bright images, since the noise is not present in these areas)

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