Signal
Free cross-plaform secure messenger-
Free Software: 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. This also implies that you can examine the source code and see what it does and how it works. This represents a baseline requirement for recommendation in this guide.
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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.
Signal uses E2EE for text, voice, and video communication.
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Limits 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.
The Signal service is designed to limit the collection and retention of metadata.
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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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Disallows Alternative Clients: Requires the use of sanctioned software clients with the provided service. This means that if the official client contains anti-features, for example, you cannot avoid them by using an alternative.
Signal has a policy of disallowing alternative or modified clients with their service, and since it represents a centralized walled garden, this makes Signal a take-it-or-leave-it proposition.
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Requires Personal Identifier: Requires personally identifiable information (PII) in order to communicate, which makes pseudonymous communication difficult or impossible.
Signal requires your phone number to register an account. However, you may choose a pseudonym as an identifier to avoid exposing your registered phone number to your contacts.