|Developer(s)||Telegram Messenger LLP</td></tr>|
|Initial release||August 2013</td></tr>|
|Stable release||Template:Latest stable software release/Telegram</td></tr>|
|Operating system||Android, Apple iOS, Windows Phone, MS Windows, Linux, OS X</td></tr>|
|Size||9.6 MB (Android), 17 MB (Windowsphone)</td></tr>|
|Available in||English, Arabic, Spanish, German, Italian, Korean, Dutch, Portuguese (Brazil & Portugal)</td></tr>|
|License||GPL v2 (client), closed source (server)</td></tr>|
|Alexa rank||Template:Increase 5,885</td></tr>|
Telegram is an instant messaging system focusing on privacy and multi-platform availability. Telegram clients exist for both mobile (Android, iOS, Windows Phone) and desktop systems (Windows, OS X, Linux). Telegram users can exchange messages encrypted end-to-end, self-destructing messages, along with photos, videos and files of any type up to 1.5GB in size.
Telegram is run by a German nonprofit organization backed by entrepreneur and philanthropist Pavel Durov. It open-sourced its client code, while server-side code is currently proprietary. Software is also available from independent developers who make use of the Telegram API.
Telegram was launched in 2013 by brothers Nikolai and Pavel Durov, the founders of Russian VK, Russia's largest social network. Telegram Messenger LLP is an independent nonprofit company based in Berlin, Germany, which is not connected to VK. Nikolai created the new MTProto protocol that the messenger is based on, while Pavel provided financial support and infrastructure through his Digital Fortress fund.
In October 2013, Telegram had 100,000 daily active users. On 24 March 2014, Telegram announced that it has reached 35 million monthly users and 15 million daily active users. In December 2014, Telegram announced they had 50 million active users, generating 1 billion daily messages and that they had 1 million new users signing up on their service every week.
Telegram runs contests challenging people to break their cryptography and disclose confidential information within a chat between two fake users. They claim that such challenges will be a permanent feature of their project. On December 21, 2013, a Russian IT-community user discovered a potential vulnerability in Telegram. They rewarded a $100,000 USD bounty after the problem was fixed. On March 1, 2014 their first cryptography contest ended with no winners and they published the keys necessary to decrypt traffic. Security researchers Moxie Marlinspike, the founder of Open Whisper Systems, and Taylor HornbyTemplate:Self-published source criticized Telegram's first cryptanalysis contest for being no proof of security and misleading the public. Subsequently a new contest which allowed a Man-in-the-middle attack was started.
As of February 17, 2015, Telegram's standard messages score 4 out of 7 points on the Electronic Frontier Foundation's secure messaging scorecard. It receives points for encryption during transit, having the code open to independent review, proper documentation of security design, and for passing a recent code audit. It misses points because communications are encrypted with a key the provider has access to, users can't verify contacts' identities, and because past messages are not secure if the encryption keys are stolen. In the same evaluation, chats in Telegram's secret chat mode scored 7 out of 7 points.
Telegram accounts are tied to the phone number of the user, with possession of the phone with that number verified with a code sent by SMS or phone call to that phone. Subsequent logins are notified to the initial device with the IP address and time of login. The phone number associated with an account can be changed without losing messages. While availing this migration, contacts of a user who have the previous phone number of the user, and are not blocked by the user, will receive the user's new number automatically. Users can set up an alias or username which can be used by any other user to contact them, without having to add them to their contacts. telegram.me is a webservice provided by Telegram which allows directly opening a chat with a user by following a hyperlink (like telegram.me/username) on supported devices.
The application features two types of chats. Ordinary chats use client-server encryption and can be accessed from multiple devices. Secret Chats use end-to-end encryption and can only be accessed from the two participating devices.
User accounts are deleted automatically after 6 months of inactivity, and this can be changed to between 1 month and 1 year.
Users can control the 'last seen' timestamps, replacing them with fudged messages like "last seen within a week". They can block other users and silence chats.
One or two checkmarks are displayed to indicate that the message is sent to the cloud or read by the recipient, respectively. It is also possible to see the status of the recipient when responding to a message.
With standard messages Telegram supports cloud messaging and file sharing (with multimedia and documents up to 1 gigabyte in size) with synchronization across devices. The message history can be searched without opening or downloading the conversations. Sharing audio notes, geo-zones, GIFs or animated memes via Giphy are also supported.
Telegram claims it is more secure than some other messengers such as WhatsApp and LINE. Conversations are encrypted with AES-256 using the MTProto protocol developed by Telegram. Only two devices may use the secret chat simultaneously.
While using secret chats in official clients, messages deleted on one device are deleted on the other device too, a special message is displayed when a screenshot is taken, and messages can be set to be deleted automatically (self-destruct) at pre-set time intervals.
On December 1, 2014, Telegram implemented Perfect Forward Secrecy in Secret Chats. This enables periodically changing the encryption keys utilized, keeping past communications safe. Official Telegram clients initiate re-keying once a key has been used to decrypt and encrypt more than 100 messages, or has been in use for more than one week, provided the key has been used to encrypt at least one message. Old keys are then discarded and cannot be reconstructed, even with access to the new keys currently in use.
All official Telegram clients (and some of the unofficial clients) are open source. Telegram's server-side software, however, is proprietary. Pavel Durov said the server code is not free software because Telegram requires a major architecture redesign to allow independent servers to act as an unified Telegram cloud.
List of applications
Telegram has various clients with support for more platforms expected in the future. Its server system is decentralized and distributed around the world to improve response times by geographical position. This list includes versions developed on official platforms backed by the development team, or supported by the community to be integrated in the future.