Facebook is rolling out end-to-end encryption for Messenger, its mobile-based chat app.
The company hinted earlier this year that it would expand its encryption efforts in the wake of the debacle between Apple and the FBI over access to an iPhone used by one of the San Bernardino shooters.
The end-to-end encrypted service, based on the trusted Signal protocol -- favored by whistleblower Edward Snowden -- prevents hackers, nation states, and anyone else from intercepting and reading messages.
A limited number of users will have access to the so-called "secret conversations" feature for iPhones, iPads, and Android devices.
But already the company has faced some criticism for not encrypting messages by default and instead making the service opt-in, like Apple's iMessage. Facebook's other chat app, WhatsApp, switched on default end-to-end encryption earlier this year.
Cryptographer and Johns Hopkins professor Matthew Green, who reviewed an early version of the system, said in a tweet you have to "turn on encryption per thread". He added that providing encryption to almost a billion people makes it hard to "put that genie back in the bottle".
The company said the reason for the opt-in was for testing and not disrupting the experience for the rest of its 900 million users.
It comes just a few months after Google was hit with a storm of criticism when it announced Allo, a similar encrypted messaging app, which users had to opt-in to take advantage of the message protection.
Snowden piled on the critique, calling the app "dangerous" and "unsafe" for not turning on encryption by default.
Facebook Chief Security Officer Alex Stamos said in a tweet that the encryption opt-in was a "small test for now", with an aim for gathering feedback down the line.
Because reading encrypted messages only works on one device at a time, the company said the experience "may not be right for everyone". Apps like WhatsApp can only work on one device at a time because that's where private keys, used to scramble messages, are stored.
Green said that it was a "pain" to move a legacy system like Messenger to full encryption, "at least in one go".
A wider rollout of the new end-to-end encrypted service is slated for later this year.