I wouldn't touch Viber with a ten foot pole, even if their entire crypto implementation was open source (which it is not). The company was founded by an ex-military (some sources say ex-Mossad) dude. He is also connected to several spyware applications.

“Talmon served for four years in the Israel Defense Forces and held the position of CIO of the central command. He graduated Cum Laude from the Tel-Aviv University with a degree in Computer Science and Management.”

More here: https://viberphoneapp.wordpress.com/

http://www.moh10ly.com/home/security/viberisaspy

http://www.jpost.com/Business/Business-Features/Japans-Rakut...

https://www.theguardian.com/technology/2013/aug/30/viber-fou...