The KW-37, code named JASON, was an encryption system developed In the 1950s by the U.S. National Security Agency to protect fleet broadcasts of the U.S. Navy. Naval doctrine calls for warships at sea to maintain radio silence to the maximum extent possible to prevent ships from being located by potential adversaries using radio direction finding. To allow ships to receive messages and orders, the navy broadcast a continuous stream of information, originally in Morse code and later using radioteletype. Messages were included in this stream as needed and could be for individual ships, battle groups or the fleet as a whole. Each ship's radio room would monitor the broadcast and decode and forward those messages directed at her to the appropriate officer. The KW-37 was designed to automate this process. It consisted of two major components, the KWR-37 receive unit and the KWT-37 transmit unit. Each ship had a complement of KWR-37 receivers (usually at least two) that decrypted the fleet broadcast and fed the output to teleprinter machines. KWT-37's were typically located at shore facilities, where high power transmitters were located. The KWR-37 weighed 100 pounds (45 kg) and contained some 500 subminiature vacuum tubes, whose leads were soldered to printed circuit boards. Each flip-flop in the KW-37 required three tubes, placing an upper bound on the total number of stages in any shift registers used at 166. Squeezing so much logic in such a small and rugged package was quite a feat in the 1950s. Each KWT-37 filled an entire relay rack with five stacked modules. A precision time reference occupied the bottom, three key generators (stream cyphers in civilian parlance) occupied the middle and an alarm panel occupied the top position. The outputs of the three key generators were combined in a voting circuit. If one of the units' output did not match the other two, an alarm was sounded and the output from the two units that did agree continued to be used. Each KWR-37 and each key generator in the KWT-37 had a common fill device (CFD) for loading keys (or as NSA calls them cryptovariables). The CFDs were similar to that first used in the KW-26, accepting punched cards in Remington Rand format. The key was changed every day at 0000 hours GMT. The receivers were synchronized to the transmitter at that time. If a receiver ever got out of sync, say due