To participate in the trial, you'll first have to complete an online form on Australia's eSafety Commissioner's official website. You will then be asked to send the images you want to block to yourself on Messenger. The commissioner's office will notify Facebook that you sent in a form -- it won't have access to your images -- so a "specially trained representative" from the social network can review and hash your images.

Whenever someone uploads pictures on Facebook, it checks them against a database of hashes, which are like digital fingerprints and are unique for each photo. Facebook and other internet titans already use the technique to fight the dissemination of child porn online: they keep a database of hashes from known child porn images in an effort to block them.

Davis assures everyone who wants to use the feature that the the company only keeps those number-and-letter hashes and not the photos themselves. Also, Facebook promises that once it's done hashing your images, you'll get a notification telling you to delete them from Messenger, so Facebook can also jettison them from its servers. At the moment, only users in Australia have access to the feature, but the social network is reportedly planning (probably depending on user feedback) to roll it out in the US, UK and Canada in the near future.