spring loading of the contacts checked as part of the routine maintenance of the device. Because the KW-26 sent a continuous stream of bits, it offered traffic-flow security. Someone intercepting the ciphertext stream had no way to judge how many real messages were being sent, making traffic analysis impossible. One problem with the KW-26 was the need to keep the receiver and transmitter units synchronized. The crystal controlled clock in the KW-26 was capable of keeping both ends of the circuit in sync for many hours, even when physical contact was lost between the sending and receiving units. This capability made the KW-26 ideally suited for use on unreliable HF radio circuits. However, when the units did get out of sync, a new key card had to be inserted at each end. The benefit of traffic-flow security was lost each time new cards were inserted. In practice, operational protocol led to the cards being replaced more often than was desirable to maintain maximum security of the circuit. This was especially so on radio circuits, where operators often changed the cards many times each day in response to a loss of radio connectivity. In any case, it was necessary to change the cards at least once per day to prevent the cypher pattern from repeating. Early KW-26 units protected the CRITICOMM network, used to protect communications circuits used to coordinate signals intelligence gathering. The initial production order for this application, awarded to Burroughs in 1957, was for 1500 units. Other services demanded KW-26's and some 14000 units were eventually built, beginning in the early 1960s, for the U.S. Navy, Army, Air Force, Defense Communications Agency, State Department and the CIA. It was provided to U.S. allies as well. When the USS Pueblo was captured by North Korea in 1968, KW-26's were on board. In response, the NSA had modifications made to other units in the field, presumably changing the crypto algorithm in some way, perhaps by changing the shift register feedback taps. Starting in the mid-1980s, the KW-26 system was decommissioned by NSA, being replaced by the more advanced solid-state data encryptor, TSEC/KG-84. See also NSA encryption systems External links KW-26 history page NSA brochure - Securing Record Communications: The TSEC/KW-26 National Security Agency encryption devices