Telegram is a cloud-based instant messaging and voice over IP service developed by Telegram Messenger LLP, a privately held company registered in London, United Kingdom, founded by Russian entrepreneur Pavel Durov and his brother Nikolai.
Telegram client apps are available for Android, iOS, Windows Phone, Windows, and MacOS. Users can send messages and exchange photos, videos, stickers, audio and files of any type. Telegram's client-side code is open-source software but 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 March 2018, Telegram stated that it had 200 million monthly active users. According to its CEO, as of April 2017, Telegram's annual growth rate was greater than 50%.Messages and media in Telegram are only client-server encrypted and stored on the servers by default.
The service provides end-to-end encryption for voice calls, and optional end-to-end encrypted "secret" chats between two online users, yet not for groups or channels.Telegram's security model has received notable criticism by cryptography experts.
They criticized the general security model of permanently storing all contacts, messages and media together with their decryption keys on its servers by default and by not enabling end-to-end encryption for messages by default. Pavel Durov has argued that this is because it helps to avoid third-party unsecure backups, and to allow users to access messages and files from any device.
Cryptography experts have furthermore criticized Telegram's use of a custom-designed encryption protocol that has not been proven reliable and secure.Telegram has faced censorship or outright bans in some countries over accusations that the app's services have been used to facilitate illegal activities, such as protests and terrorism, as well as declining demands to facilitate government access to user data and communications.