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A warrant canary is a method used by a service provider to inform their customers that the provider has not been served with a secret government subpoena. Such subpoenas, including those covered under the USA Patriot Act, provide criminal penalties for revealing the warrant to a third party, such as the customers. A warrant canary may be posted by the provider to inform customers of dates that they haven't been served a secret subpoena. If the canary has not been updated in the time period specified by the host, customers are to assume that they have been served with such a subpoena. Theoretically, this allows the provider to inform customers of subpoenas without violating any laws. The legality of this has not been tested in any court, however, and the results of the Doe v. Ashcroft lawsuit as well as subsequent changes to the USA PATRIOT Act may obviate the tool for the time being.
A warrant canary may include a digital signature as proof that the message was written by the hosting provider.
The idea of such a negative pronouncement being used to thwart secret warrants was first proposed on the cypherpunks mailing list, and was first implemented by public libraries in response to the USA Patriot Act.