Telegram, an open-source competitor of WhatsApp
First of all Telegram is free and open-source, and you can grab the source from here. Well known security protocols are open-source and this gives the possibility for communities of cryptographers, hackers and public audience to test their actual security. Using two layers of secure encryption with 256-bit symmetric AES encryption, RSA 2048 encryption and Diffie–Hellman secure key exchange. It’s impossible to brute force a RSA 2048 encryption key with all the computers available on the universe.