AMITIAE - Tuesday 22 July 2014
Cassandra: Identifying Images on Social Networking Sites Using Sensor Pattern Noise (SPN)
By Graham K. Rogers
While some metadata could be used for identification of camera or user, this is easily removed in post-processing, so may not always be available.
The authors Riccardo Satta and Pasquale Stirparo explain that light goes through the lens, then through an anti-aliasing filter. It reaches reaches after passing through a Colour Filter Array (CFA). There are more steps after the senson including de-mosaicking and post-processing. They explain that "Each step of this pipeline may leave artefacts on the image that can be used as a signature of the camera device."
Dust mainly affects cameras with interchangeable lenses and could identify a single device. This may not always be valid, however, especially as lenses may be switched between cameras and even sold on; and dust deposits change over time (most photographers will also clean the lenses). However, specific lens "aberrations" may help in identifications. This may be more useful in terms of smartphone use as lenses are not interchangeable.
However, research has shown that Sensor Pattern Noise has particular "characteristics of uniqueness and stability". Once a particular set of characteristics have been identified using the algorithms developed, they can be matched against images, perhaps including those uploaded to social networking sites.
As was found in the case of Mr Swirly Man (Christopher Paul Neil) who was finally arrested in Bangkok, once identifications are made, finding the persons and questioning them are likely to reveal more information and bring a case to a conclusion.
While identifications did not approach 100%, which would be desirable for forensic accuracy, the probability (above 50%) would point to the need for further inquiries, including matching information from other sources.
The researchers were particularly interested in the use of this means of identifying offenders in cases of "On-line Child Abuse, cyber- bullying, or thefts of smart-phones." Further research will seek to enhance the ways in which identification by means of SPN could be used.
See also:On the usage of Sensor Pattern Noise for Picture-to-Identity linking through social network accounts (Riccardo Satta and Pasquale Stirparo).
Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand where he is also Assistant Dean. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs. He is now continuing that in the Bangkok Post supplement, Life.
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