I highly recommend following the updates from Frederic Jacobs on his Twitter. He has already found a number of significant flaws with Viber encryption...

"Wow, Viber disables crypto based on geo-location? Export controls related?" https://twitter.com/FredericJacobs/status/722511416381480961

"Viber’s encryption appears to be a custom C++ implementation. Super reassuring they use MD5 for attachments." pic.twitter.com/wi6lB30KjY

https://twitter.com/FredericJacobs/status/722489499779858432