It's a free cloud-based instant messaging service. Telegram clients exist for both mobile (Android, iOS, Windows Phone, Ubuntu Touch) and desktop systems (Windows, macOS, Linux). Users can send messages and exchange photos, videos, stickers, audio, and files of any type. Telegram also provides optional end-to-end-encrypted messaging. Telegram is supported by Russian entrepreneur Pavel Durov. Its client-side code is open-source software but contains binary blobs, and the source code for recent versions is not always immediately published, whereas its server-side code is closed-source and proprietary. The service also provides APIs to independent developers. In February 2016, Telegram stated that it had 100 million monthly active users, sending 15 billion messages per day. The security of Telegram has faced notable scrutiny; critics have claimed that Telegram's security model is undermined by its use of a custom-designed encryption protocol that has not been proven reliable and secure, and by not enabling secure conversations by default. According to its CEO, as of April 2017, Telegram has 40 million monthly active users in Iran, surpassing other messaging apps in the country.