Telegram is a cloud-based instant messaging service, supported by the Russian-born entrepreneur Pavel Durov. It also offers optional end-to-end encrypted Secret Chat and self-destruct messages.
Whereas its server-side code is closed-sourced and proprietary, its client-side code is open-source. Telegram provides APIs to independent developers.
The MTProto protocol is based on 256-bit symmetric AES encryption, RSA 2048 encryption, and Diffie–Hellman key exchange.