Telegram is a cloud-based instant messaging and voice over IP service. Telegram client apps are available for Android, iOS, Windows Phone, Windows NT, macOS and Linux. 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.
Messages and media in Telegram are encrypted when stored on its servers, and the client-server communication is also encrypted. 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.
Development
Telegram was launched in 2013 by the brothers Nikolai and Pavel Durov. Previously the pair founded the Russian social network VK, which they left when it was taken over by the Mail.ru Group. Nikolai Durov created the MTProto protocol that is the basis for the messenger, while Pavel provided financial support and infrastructure through his Digital Fortress fund with partner Axel Neff joining as a second co-founder. Telegram Messenger LLP states that its end goal is not to bring profit, but it is not currently structured as a non-profit organization.
Telegram is registered as both an English LLP and an American LLC. It does not disclose where it rents offices or which legal entities it uses to rent them, citing the need to "shelter the team from unnecessary influence" and protect users from governmental data requests. Pavel Durov has said that the service was headquartered in Berlin, Germany, between 2014 and early 2015, but moved to different jurisdictions after failing to obtain residence permits for everyone on the team. Durov left Russia and is said to be moving from country to country with a small group of computer programmers consisting of 15 core members. According to press reports, Telegram had employees in St. Petersburg. The Telegram team is currently based in Dubai.
Usage numbers
In October 2013, Telegram had 100,000 daily active users. On 24 March 2014, Telegram announced that it had reached 35 million monthly users and 15 million daily active users. In October 2014, South Korean governmental surveillance plans drove many of its citizens to switch to Telegram. In December 2014, Telegram announced that it had 50 million active users, generating 1 billion daily messages, and that it had 1 million new users signing up on its service every week; traffic doubled in five months with 2 billion daily messages. In September 2015, an announcement stated that the app had 60 million active users and delivered 12 billion daily messages. In February 2016, Telegram announced that it had 100 million monthly active users, with 350,000 new users signing up every day, delivering 15 billion messages daily. In December 2017, Telegram reached 180 million monthly active users. In March 2018, Telegram reached 200 million monthly active users. On March 14, 2019, Pavel Durov claimed that “3 million new users signed up for Telegram within the last 24 hours.” Durov didn't specify what prompted this flood of new sign-ups, but the period matched a prolonged technical outage experienced by Facebook and its family of apps, including Instagram.
According to the U.S. Securities and Exchange Commission, the number of monthly Telegram users as of October 2019 is 300 million people worldwide.
Account
Account self-destruction
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Telegram accounts are tied to telephone numbers and are verified by SMS. Users can add multiple devices to their account and receive messages on each one. Connected devices can be removed individually or all at once. The associated number can be changed at any time and when doing so, the user's contacts will receive the new number automatically. In addition, a user can set up an alias that allows them to send and receive messages without exposing their phone number. Telegram accounts can be deleted at any time and they are deleted automatically after six months of inactivity by default, which can optionally be changed to 1 month and 12 months. Users can replace exact "last seen" timestamps with broader messages such as "last seen recently".
The default method of authentication that Telegram uses for logins is SMS-based single-factor authentication. All that is needed in order to log into an account and gain access to that user's cloud-based messages is a one-time passcode that is sent via SMS to the user's phone number. These login SMS messages are known to have been intercepted in Iran, Russia and Germany, possibly in coordination with phone companies. Pavel Durov has said that Telegram users in "troubled countries" should enable two-factor authentication by creating passwords, which Telegram allows, but does not require.
Cloud-based messages
Telegram's default messages are cloud-based and can be accessed on any of the user's connected devices. Users can share photos, videos, audio messages and other files (up to 1.5 gigabyte in size per file). Users can send messages to other users individually or to groups of up to 100,000 members. Sent messages can be edited and deleted on both sides within 48 hours after they have been sent. This gives user an ability to correct typos and retract messages that were sent by mistake. The transmission of messages to Telegram Messenger LLP's servers is encrypted with the service's MTProto protocol. According to Telegram's privacy policy, "all data is stored heavily encrypted and the encryption keys in each case are stored in several other DCs in different jurisdictions. This way local engineers or physical intruders cannot get access to user data". This makes the messages' security roughly comparable to that of e-mail. Here, most providers employ client-server encryption as well, however usually with the standardized protocol Transport Layer Security. E-mails may or may not be encrypted on the servers. Telegram cloud messages and media remain on the servers at least until deleted by all participants.
Bots
In June 2015, Telegram launched a platform for third-party developers to create bots. Bots are Telegram accounts operated by programs. They can respond to messages or mentions, can be invited into groups and can be integrated into other programs. It also accepts online payments with credit cards and Apple Pay. Dutch website Tweakers reported that an invited bot can potentially read all group messages when the bot controller changes the access settings silently at a later point in time. Telegram pointed out that it considered implementing a feature that would announce such a status change within the relevant group. Also there are inline bots, which can be used from any chat screen. In order to activate an inline bot, user needs to type in the message field a bot's username and query. The bot then will offer its content. User can choose from that content and send it within a chat.
Channels
In September 2015, Telegram added channels. Channels are a form of one-way messaging where admins are able to post messages but other users are not. Any user is able to create and subscribe to channels. Channels can be created for broadcasting messages to an unlimited number of subscribers. Channels can be publicly available with an alias and a permanent URL so anyone can join. Users who join a channel can see the entire message history. Users can join and leave channels at any time. Depending on a channel's settings, messages may be signed with the channel's name or with the username of the admin who posted them. Non-admin users are unable to see other users who've subscribed to the channel. Furthermore, users can mute a channel, meaning that the user will still receive messages, but won't be notified. Admins can give permission to post comments on the Telegram channel with help of bots. The admin of the channel can obtain general data about the channel. Each message has its own view counter, showing how many users have seen this message. As of May 2019, the creator of a channel can add a discussion group, a separate group where messages in the channel are automatically posted for subscribers to communicate.
Stickers
Stickers are cloud-based, high-definition images intended to provide more expressive emoji. When typing in an emoji, the user is offered to send the respective sticker instead. Stickers come in collections called "sets", and multiple stickers can be offered for one emoji. Telegram comes with one default sticker set, but users can install additional sticker sets provided by third-party contributors. Sticker sets installed from one client become automatically available to all other clients. Sticker images use WebP file format, which is better optimized to be transmitted over internet.
Drafts
Drafts are unfinished messages synced across user devices. One can start typing a message on one device and continue on another. The draft will persist in editing area on any device until it is sent or removed.
Secret chats
Messages can also be sent with client-to-client encryption in so-called secret chats. These messages are encrypted with the service's MTProto protocol. Unlike Telegram's cloud-based messages, messages sent within a secret chat can be accessed only on the device upon which the secret chat was initiated and the device upon which the secret chat was accepted; they cannot be accessed on other devices. Messages sent within secret chats can, in principle, be deleted at any time and can optionally self-destruct.
Secret chats have to be initiated and accepted by an invitation, upon which the encryption keys for the session are exchanged. Users in a secret chat can verify that no man-in-the-middle attack has occurred by comparing pictures that visualize their public key fingerprints.
According to Telegram, secret chats have supported perfect forward secrecy since December 2014. Encryption keys are periodically changed after a key has been used more than 100 times or has been in use for more than a week. Old encryption keys are destroyed.
Windows and Linux users are still not able to use secret chats using the official Telegram Desktop app while the official macOS-only client supports them.
Secret chats are not available for groups or channels.
Telegram's local message database is not encrypted by default. Some Telegram clients allow users to encrypt the local message database by setting a passphrase.
Voice calls
In the end of March 2017, Telegram introduced its own voice calls. The calls are built upon the end-to-end encryption of Secret Chats. Connection is established as peer-to-peer whenever possible, otherwise the closest server to the client is used. According to Telegram, there is a neural network working to learn various technical parameters about call to provide better quality of the service for future uses. After a brief initial trial in Western Europe, voice calls are now available for use in most countries.
Telescope (video messages)
Since version 4.0, released in May 2017, Telegram offers a dedicated video hosting platform called Telescope. The round videos can be up to one minute long and autoplay. When posted in a public channel on Telegram, the videos are also uploaded to and viewable without an account at telesco.pe. However, Telegram video messages and "Telescope" videos sent within non-public chats or groups are not published.
Live locations
For either 15 minutes, one hour, or eight hours, Telegram users can share their live location in a chat since version 4.4 released in October 2017. If multiple users share their live location within a group, they are shown on an interactive map. Sharing the 'live location' can be stopped at any time.
Social login
In February 2018, Telegram launched their social login feature to its users, named as Telegram Login. It features a website widget that could be embedded into websites, allowing users to sign into a third party website with their Telegram account. The gateway sends users' Telegram name, username, and profile picture to the website owner, while users' phone number remains hidden. The gateway is integrated with a bot, which is linked with the developer's specific website domain.
Passport
In July 2018, Telegram introduced their online authorisation and identity management system, Telegram Passport, for platforms that require real-life identification. It asks users to upload their own official documents such as passport, identity card, driver license, etc. When an online service requires such identification documents and verification, it forwards the information to the platform with the user's permission. Telegram stated that it does not have access to the data, while the platform will only share the information to the authorised recipient. However, the service was criticised for being vulnerable to online brute force attacks.
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