— Bytemark Hosting (@bytemark) June 29, 2013
A warrant canary is a method used by an Internet service provider to inform its 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 existence of the warrant to any third party, including the service provider’s 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 the host has been served with such a subpoena. The intention is to allow the provider to inform customers of the existence of a subpoena passively, without violating any laws. The legality of this method has not been tested in any court.
The idea of using negative pronouncements to thwart secret warrants was first proposed by Steven Schear on the cypherpunks mailing list, and was first implemented by public libraries in response to the USA Patriot Act.
The first commercial use of a warrant canary was by rsync.net. In addition to a digital signature, they provide a recent news headline as proof that the warrant canary was recently posted as well as mirroring the posting internationally.