Messages and media in Telegram are only server-client encrypted by default. Cryptography experts have furthermore criticized Telegram's use of a custom-designed encryption protocol that has not been proven reliable and secure. 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. 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. Telegram's security model has received notable criticism by cryptography experts. According to its CEO, as of April 2017, Telegram has more than 50% annual growth rate.

In March 2018, Telegram stated that it had 200 million monthly active users. The service also provides APIs to independent developers. 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. Users can send messages and exchange photos, videos, stickers, audio and files of any type. Telegram client apps are available for Android, iOS, Windows Phone, Windows NT, macOS and Linux. Telegram is a cloud-based instant messaging service developed by Telegram Messenger LLP, a privately held company registered in London, United Kingdom, founded by the Russian entrepreneur Pavel Durov.

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