End-to-end encryption has become an increasingly important feature for messaging platforms as people become more and more concerned about security. Now, Facebook Messenger is jumping in on the trend with a new feature called "secret conversations". It won't be enabled by default, however. The feature will be opt-in and for now it's available only to a small number of users for testing, with a wider rollout promised for later this summer or in early fall.

Messenger's encryption will be based on Open Whisper System's Signal Protocol – the same well-known and well-tested encryption that Facebook-owned WhatsApp uses.

Besides the fact that users will need to choose to turn on secret conversations manually, there are a few other limitations to how Facebook is implementing end-to-end encryption. For one thing, "secret conversations" will only work from one single phone or tablet selected by the user – instead of across all your devices. It also won't cover "rich content" like GIFs. Facebook explains that end-to-end encryption needs to remain optional so that users can access other popular Messenger features, such as chatbots and payment features.

If you're part of Facebook's test group, you'll see an option called "Secret Conversation" when tapping a friend's name at the top of your current message thread. Tapping on that option will launch a new conversation thread with a notice at the top informing you that the chat is end-to-end encrypted. This means – in theory – that someone snooping on your network, a government agency with a warrant, or even Facebook itself can't intercept messages. Users will also have the option of setting self-destruct timers on messages.