content: According to Wikipedia,   The idea of using negative pronouncements to thwart the nondisclosure requirements of court orders and served secret warrants was first proposed by Steven Schear on the cypherpunks mailing list,[7]  and that was in October 2002. So long as you only say "We have not gotten a subpoena," and stop saying it when you do get a subpoena, then it appears warrant canaries have been upheld in court.