It is assumed that to identify prohibited content Apple will use the technology PhotoDNA, which was developed by Microsoft back in 2009. The cupertinos never report on what stage they check user photos. Orr believes that this may occur at the time of transmission of images on the server, since after the download is complete, photos are stored in encrypted form.
Overall nothing wrong with such systems of check no. Today, many large companies, including Facebook and Twitter, check the content that users upload to their pages. However, confuses that fact, that cupertinos not shared additional details on what specifically the content is subject to verification.