Apple Inc on Thursday said it will implement a system that checks photos on iPhones in the United States before they are uploaded to its iCloud storage services to ensure the upload does not match known images of child sexual abuse. Detection of child abuse image uploads sufficient to guard against false positives will trigger a human review of and report of the user to law enforcement, Apple said. It said the system is designed to reduce false positives to one in one trillion. Apple’s new system seeks to address requests from law enforcement to help stem child sexual abuse while also respecting privacy and security practices that are a core tenet of the company’s brand. But some privacy advocates said the system could open the door to monitoring of political speech or other content on iPhones. Most other major technology providers are already checking images against a database of known child sexual abuse imagery. Here is how Apple’s system works. Law enforcement officials maintain a database of known child sexual abuse images and translate those images into “hashes” — numerical codes that positively identify the image but cannot be used to reconstruct them.