Telegram
Free cross-plaform messenger and chat service-
Proprietary: Free (as in freedom, not price) software puts the user in control by allowing you to use the software for any purpose, modify it, and redistribute it. Non-free or proprietary software, on the other hand, witholds one or more of these freedoms, giving the developer of the software unjust power over the users.
Telegram describes its official mobile apps as “open source” on its website, but this does not appear to be the case, as Telegram does not make the source code for the iOS app available under a free (or any) license.
The official Android application and desktop application are licensed under the GPLv2 and GPLv3, respectively, and are therefore free software.
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End-to-End Encryption (E2EE): Allows two or more end-points to communicate confidentially, such that no one in the middle (including the service operator) has access to the contents of the communication. This represents a baseline requirement for recommendation in this guide.
Telegram secures video and voice calls with E2EE, while only chats designated as “secret” benefit from E2EE. The official Windows and GNU/Linux clients (Telegram Desktop) lack support for participating in “secret” chats altogether. Furthermore, Telegram has drawn criticism on many fronts from cryptography experts, largely stemming from a history of exaggerated, unverified, or false claims made by Telegram about their security model.
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Does Not Limit Metadata: Metadata is "data about data." In the context of communication, metadata could represent information about who sent or received a message and at what date and time.
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Centralized: Centralized services are operated and controlled by a single entity, putting the operator in the position of gatekeeper with total power over your use of the service. Centralized systems also suffer from a single point of failure and are vulnerable to censorship both by the operator and by governments with jurisdiction over the operation of the service.
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Limited Interoperability: Interoperability increases freedom and usefulness by allowing different systems to work together. When a service is hostile to other services interoperating, particularly a communication service, this places users in a walled garden, which limits choice and can make it difficult to switch to or from an alternative.
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Allows Alternative Clients: Does not require the use of sanctioned software clients with the provided service. This means that if the official client contains anti-features, for example, you can easily avoid them by using an alternative.
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Requires Personal Identifier: Requires personally identifiable information (PII) in order to communicate, which makes pseudonymous communication difficult or impossible.
Telegram accounts must be registered with a telephone number and verified via SMS. Registering addition devices requires verification via SMS, as well. Phone numbers are shared with contacts unless the user sets up a username as an alias.