A police investigation into shadowy groups on Google Hello and the Multiply social network has revealed that pedophiles are using powerful encryption tools in social media and other programs to illegally share child pornography. Child-porn rings are also using a simple — but highly effective — tool to keep prying investigators at bay. Would-be ring members are increasingly being asked to share photographs and videos of children being sexually abused in order to gain entry, investigators say. Because sharing child pornography is a crime — it re-victimizes abused children — law enforcement officers are prohibited from offering images and videos in sting operations. As a result, officials say, it is becoming more difficult to monitor child-porn rings.