The Staatstrojaner ("Federal Trojan horse") is a computer surveillance program installed secretly on a suspect's computer, which the German police uses to wiretap Internet telephony. This "source wiretapping" is the only feasible way to wiretap in this case, since Internet telephony programs will usually encrypt the data when it leaves the computer. The Federal Constitutional Court of Germany has ruled that the police may only use such programs for telephony wiretapping, and for no other purpose, and that this restriction should be enforced through technical and legal means.