Comment 7 for bug 1690832

Please note that using data-dependent operations in an attempt to obfuscate data is extremely dangerous:

http://fusion.kinja.com/um-bad-news-pixelating-or-blurring-doesnt-actually-wo-1793860362
https://dheera.net/projects/blur
http://cseweb.ucsd.edu/~saul/papers/pets16-redact.pdf

If you want to keep part of an image private use something that does not depend upon the underlying data. Be sure that you're not simply applying a black bar on a different layer, or changing colors of text or background, as those are also prone to failure. (Not really applicable to png but other file formats have multiple layers.)

http://blogs.adobe.com/security/2009/12/how_to_properly_redact_pdf_fil.html
https://apple.stackexchange.com/questions/22683/blacking-out-a-part-of-a-pdf-or-redaction-of-text

Thanks