The company, however, says that they’re doing “open source plus,” but have not yet named what open source tools it’s pulling from:

Viber doesn’t say what it’s using, leading some to speculate that the company tried to roll its own (generally not a good idea - and likely means there are serious security flaws). With Whatsapp, the company was upfront in saying that it was using the popular and tested open source encryption from Open Whisper Systems. Unfortunately, Viber is not entirely clear on what encryption tools they’re using. The company claims that the encrypted system will also let you know if your content is encrypted based on color coding. It’s already testing encryption in a few markets, before rolling it out globally. Following Whatsapp’s big move to roll out end-to-end encryption, the super popular communications app Viber has announced it intends to do the same for its 700 million (and growing) users. It appears that more fully encrypting messaging and content is really catching on. Thu, Apr 21st 2016 03:33pm - Mike Masnick

Is viber encrypted now