It has long been speculated, and was argued in court in one case, that these stations operate as a simple and foolproof method for government agencies to communicate with spies working undercover. According to this theory, the messages are encrypted with a one-time pad, to avoid any risk of decryption by the enemy. As evidence, numbers stations have changed details of their broadcasts or produced special, nonscheduled broadcasts coincident with extraordinary political events, such as the attempted coup of August 1991 in the Soviet Union.