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iPhone Photo's CSAM Feature Explained

Apple is introducing new child safety features in three areas, developed in collaboration with child safety experts. First, new communicat...

Apple is introducing new child safety features in three areas, developed in collaboration with child safety experts. First, new communication tools will enable parents to play a more informed role in helping their children navigate communication online. 

The Messages app will use on-device machine learning to warn about sensitive content while keeping private communications unreadable by Apple. Next, iOS and iPadOS will use new applications of cryptography to help limit the spread of CSAM online, while designing for user privacy. CSAM detection will help Apple provide valuable information to law enforcement on collections of CSAM in iCloud Photos. 

Finally, updates to Siri and Search provide parents and children expanded information and help if they encounter unsafe situations. Siri and Search will also intervene when users try to search for CSAM-related topics. These features are coming later this year in updates to iOS 15, iPadOS 15, watchOS 8, and macOS Monterey. This program is ambitious, and protecting children is an important responsibility. These efforts will evolve and expand over time.

Safety in Messages

The Messages app will add new tools to warn children and their parents when receiving or sending sexually explicit photos. When receiving this type of content, the photo will be blurred and the child will be warned, presented with helpful resources, and reassured it is okay if they do not want to view this photo. As an additional precaution, the child can also be told that to make sure they are safe, their parents will get a message if they do view it. Similar protections are available if a child attempts to send sexually explicit photos. The child will be warned before the photo is sent, and the parents can receive a message if the child chooses to send it.

Messages uses on-device machine learning to analyze image attachments and determine if a photo is sexually explicit. The feature is designed so that Apple does not get access to the messages. This feature is coming in an update later this year to accounts set up as families in iCloud for iOS 15, iPadOS 15, and macOS Monterey. Messages will warn children and their parents when receiving or sending sexually explicit photos.

CSAM detection

Another important concern is the spread of Child Sexual Abuse Material (CSAM) online. CSAM refers to content that depicts sexually explicit activities involving a child. To help address this, new technology in iOS and iPadOS;will allow Apple to detect known CSAM images stored in iCloud Photos. This will enable Apple to report these instances to the National Center for Missing and Exploited Children (NCMEC). NCMEC acts as a comprehensive reporting center for CSAM and works in collaboration with law enforcement agencies across the United States.

Apple’s method of detecting known CSAM is designed with user privacy in mind. Instead of scanning images in the cloud, the system performs on-device matching using a database of known CSAM image hashes provided by NCMEC and other child safety organizations. Apple further transforms this database into an unreadable set of hashes that is securely stored on users’ devices.

Before an image is stored in iCloud Photos, an on-device matching process is performed for that image against the known CSAM hashes. This matching process is powered by a cryptographic technology called private set intersection, which determines if there is a match without revealing the result. The device creates a cryptographic safety voucher that encodes the match result along with additional encrypted data about the image. This voucher is uploaded to iCloud Photos along with the image.

Using another technology called threshold secret sharing, the system ensures the contents of the safety vouchers cannot be interpreted by Apple unless the iCloud Photos account crosses a threshold of known CSAM content. The threshold is set to provide an extremely high level of accuracy and ensures less than a one in one trillion chance per year of incorrectly flagging a given account.

Only when the threshold is exceeded does the cryptographic technology allow Apple to interpret the contents of the safety vouchers associated with the matching CSAM images. Apple then manually reviews each report to confirm there is a match, disables the user’s account, and sends a report to NCMEC. If a user feels their account has been mistakenly flagged they can file an appeal to have their account reinstated.

This innovative new technology allows Apple to provide valuable and actionable information to NCMEC and law enforcement regarding the proliferation of known CSAM. And it does so while providing significant privacy benefits over existing techniques since Apple only learns about users’ photos if they have a collection of known CSAM in their iCloud Photos account. Even in these cases, Apple only learns about images that match known CSAM.

Expanding guidance in Siri and Search

Apple is also expanding guidance in Siri and Search by providing additional resources to help children and parents stay safe online and get help with unsafe situations. For example, users who ask Siri how they can report CSAM or child exploitation will be pointed to resources for where and how to file a report. Siri and Search are also being updated to intervene when users perform searches for queries related to CSAM. These interventions will explain to users that interest in this topic is harmful and problematic, and provide resources from partners to get help with this issue.

These updates to Siri and Search are coming later this year in an update to iOS 15, iPadOS 15, watchOS 8, and macOS Monterey. Siri will provide resources and help around searches related to CSAM.

iPhone has provided more information about these features in the documents below, including technical summaries, proofs, and independent assessments of the CSAM-detection system from cryptography and machine learning experts.