Secret broadcast

A secret broadcast is, simply put, a broadcast that is not for the consumption of the general public. The invention of the wireless was initially greeted as a boon by armies and navies. Units could now be coordinated by nearly instant communications. It soon became clear that radio was a two edged sword. An adversary could glean valuable and sometimes decisive intelligence from intercepted radio signals:

* messages that were not encrypted or poorly encrypted could be read
* order of battle and future intentions could be deduced by traffic analysis
* individual units could be located using direction finding

In the 1920s the United States was able to track Japanese fleet exercises even through fog banks by monitoring their radio transmissions.

A doctrine was developed of having units in the field, particularly ships at sea, maintain radio silence except for urgent situations, such as reporting contact with enemy forces. Ships in formation reverted to pre-wireless methods, including semaphore, signal flags, with signal lamps used at night. Communication from headquarters were sent by one-way radio broadcasts.

U.S. Fleet Broadcast

For example, the U.S. Navy developed a series of transmitting sites throughout the world that sent a fleet broadcast to its vessels at sea. Initially these were Morse code stations. Later radioteletype was used. Sensitive messages were encrypted. Call signs were encrypted as well to provide traffic-flow security. Some messages (weather forecasts, intelligence reports) were sent to the fleet as a whole. Operators on-board ship would monitor the fleet broadcast, decoding messages intended for their ships. Messages might be repeated several times on different frequencies to ensure reception, but were generally not acknowledged. During and after World War II rotor machine encryptors, such as SIGABA and KL-7 were used to decode messages off-line. In the 1960s in-line teletype decoders, such as the KW-37, were introduced. These were replaced in the 1980s by the Fleet Satellite Communications System and the KW-46 encryption system, which supported speeds up to 9600 bit/s. These in turn have been replaced with units offering the even higher speeds needed for "network centric" warfare and the dissemination of high bandwidth intelligence information, such as satellite imagery.

"Personal messages" on propaganda stations

During WWII, the BBC would include "personal messages" in its broadcasts of news and entertainment to occupied-Europe. Often they were coded messages intended for secret agents. Leo Marks attributes this idea to Georges Bégué, an agent for the Special Operations Executive who felt their use could eliminate a lot of the two-way radio traffic that often compromised agents. Such messages were also used to authenticate agents to sources of assistance in the field. The agent would arrange to have the BBC broadcast any short phrase the other person chose.

Number stations

In the mid-twentieth century, the High Frequency radio bands were used by numerous stations sending seemingly random Morse code, usually in five-letter groups. As more advanced communications methods, such as teletype and satellite, took over, the number of such stations diminished, but another type appeared that transmitted spoken and also seemingly random number and letter groups, the later usually using words from a radio alphabet such as ICAO/NATO alphabet.

Though there has been no official confirmation (beyond one comment from a British official saying "they are what you think they are") there is little doubt that most of these number stations are primarily used to send messages to spies and other clandestine agents (additional possible uses include communication with embassies when a crisis might dictate destruction of cryptographic equipment and as a backup to normal command systems in wartime). Other intended recipients of secret broadcasts have faster and easier-to-use equipment at their disposal. But number stations are ideal for spies in that they require no special equipment, beyond a short-wave receiver. Morse code skills, once a staple of spy training, are no longer required.

Problems with secret broadcast

Messages sent via secret broadcast are not acknowledged. One example where this caused disaster was the USS Liberty incident where an order removing the "Liberty" from proximity to hostile fire was not transmitted via the proper fleet broadcast station until after the ship came under attack. Another issue in the past has been the limited bandwidth of the broadcast. Morse code was typically sent at 25 words per minute. Teletype could operate at 60 words per minute. The military uses a message precedence system to prioritize critical traffic, but all too often, senior commanders insisted on high precedence for lengthy messages lacking real urgency.

ources

* David Kahn, "The Codebreakers"
* Leo Marks, "Between Silk and Cyanide"
* U.S.Cryptologic History Report, "Attack on a SIGINT Collector, the U.S.S. Liberty," U.S. National Security Agency, 1981 [http://www.nsa.gov/liberty/liber00010.pdf]

ee also

* Letter beacon
* Number station
* Traffic flow security