  In the last decade, Social Networks (SNs) have deeply changed many aspects of
society, and one of the most widespread behaviours is the sharing of pictures.
However, malicious users often exploit shared pictures to create fake profiles
leading to the growth of cybercrime. Thus, keeping in mind this scenario,
authorship attribution and verification through image watermarking techniques
are becoming more and more important. In this paper, firstly, we investigate
how 13 most popular SNs treat the uploaded pictures, in order to identify a
possible implementation of image watermarking techniques by respective SNs.
Secondly, on these 13 SNs, we test the robustness of several image watermarking
algorithms. Finally, we verify whether a method based on the Photo-Response
Non-Uniformity (PRNU) technique can be successfully used as a watermarking
approach for authorship attribution and verification of pictures on SNs. The
proposed method is robust enough in spite of the fact that the pictures get
downgraded during the uploading process by SNs. The results of our analysis on
a real dataset of 8,400 pictures show that the proposed method is more
effective than other watermarking techniques and can help to address serious
questions about privacy and security on SNs.
