Numbers station
A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalize numbers, although digital modes such as phase-shift keying and frequency-shift keying, as well as Morse code transmissions, are not uncommon. Most stations have set time schedules or schedule patterns; however, some appear to have no discernible pattern and broadcast at random times. Stations may have set frequencies in the high-frequency band.
Numbers stations have been reported since at least the start of World War I and continue in use today. Amongst amateur radio enthusiasts, there is an interest in monitoring and classifying numbers stations, with many being given nicknames to represent their quirks and features or origins.
History
According to the notes of The Conet Project, which has compiled recordings of these transmissions, number stations have been reported since with the numbers transmitted in Morse code. It is reported that Archduke Anton of Austria in his youth during World War I used to listen in to their transmissions, writing them down and passing them on to the Austrian military intelligence.Numbers stations were most abundant during the Cold War era. According to an internal Cold War-era report of the Polish Ministry of the Interior, numbers stations DCF37 and DFD21 were transmitted from West Germany beginning in the early 1950s.
Many stations from this era continue to broadcast and some long-time stations may have been taken over by different operators. The Czech Ministry of the Interior and the Swedish Security Service have both acknowledged the use of numbers stations by Czechoslovakia for espionage, with declassified documents proving the same. Few QSL responses have been received from numbers stations by shortwave listeners who sent reception reports to stations that identified themselves or to entities the listeners believed responsible for the broadcasts, which is the expected behaviour of a non-clandestine station.
One well-known numbers station was the E03 "Lincolnshire Poacher", which is thought to have been run by the British Secret Intelligence Service. It was first broadcast from Bletchley Park in the mid-1970s but later was broadcast from RAF Akrotiri in Cyprus. It ceased broadcasting in 2008.
In 2001, the United States tried the Cuban Five on the charge of spying for Cuba. The group had received and decoded messages that had been broadcast from the "Atención" number station in Cuba.
Atención spy case
The "Atención" station of Cuba became the world's first numbers station to be officially and publicly accused of transmitting to spies. It was the centerpiece of a United States federal court espionage trial, following the arrest of the Wasp Network of Cuban spies in 1998. The U.S. prosecutors claimed the accused were writing down number codes received from Atención, using Sony hand-held shortwave receivers, and typing the numbers into laptop computers to decode spying instructions. The FBI testified that they had entered a spy's apartment in 1995, and copied the computer decryption program for the Atención numbers code. They used it to decode Atención spy messages, which the prosecutors unveiled in court.The United States government's evidence included the following three examples of decoded Atención messages.
- "prioritize and continue to strengthen friendship with Joe and Dennis"
- "Under no circumstances should German nor Castro fly with BTTR or another organization on days 24, 25, 26 and 27."
- "Congratulate all the female comrades for International Day of the Woman."
21st century cases
In 2001, Ana Belén Montes, a senior US Defense Intelligence Agency analyst, was arrested and charged with espionage. The federal prosecutors alleged that Montes was able to communicate with the Cuban Intelligence Directorate through encoded messages, with instructions being received through "encrypted shortwave transmissions from Cuba".In 2006, Carlos Alvarez and his wife, Elsa, were arrested and charged with espionage. The U.S. District Court for the Southern District of Florida stated that "defendants would receive assignments via shortwave radio transmissions".
In June 2009, the United States similarly charged Walter Kendall Myers with conspiracy to spy for Cuba, and receiving and decoding messages broadcast from a numbers station operated by the Cuban Intelligence Directorate to further that conspiracy. As discovered by the FBI up to 2010, one way that Russian agents of the Illegals Program were receiving instructions was via coded messages on shortwave radio.
It has been reported that the United States has used number stations to communicate encoded information to persons in other countries. There are also claims that State Department-operated stations, such as KKN50 and KKN44, used to broadcast similar "numbers" messages or related traffic, although these radio stations have been off the air for many years.
North Korea revived number broadcasts in July 2016 after a hiatus of sixteen years, a move which some analysts speculated was psychological war; sixteen such broadcasts occurred in 2017, including unusually timed transmissions in April.
Suspected use for espionage
It has long been speculated, and was argued in one court case, that these stations operate as a simple and fool-proof method for government agencies to communicate with spies working undercover. According to this hypothesis, the messages must have been encrypted with a one-time pad to avoid any risk of decryption by the enemy. Writing in 2008, Wallace and Melton described how numbers stations could be used in this way for espionage:Evidence to support this theory includes the fact that 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.
A 1998 article in The Daily Telegraph quoted a spokesperson for the Department of Trade and Industry as saying
Formats
Generally, numbers stations follow a basic format, although there are many differences in details between stations. Transmissions usually begin on the hour or half-hour.The prelude, introduction, or call-up of a transmission includes some kind of identifier, for the station itself, the intended recipient, or both. This can take the form of numeric or radio-alphabet "code names", characteristic phrases, and sometimes musical or electronic sounds. Sometimes, as in the case of radio-alphabet stations, the prelude can also signify the nature or priority of the message to follow. Often the prelude repeats for a period before the body of the message begins.
After the prelude, there is usually an announcement of the number of number-groups in the message, the page to be used from the one-time pad, or other pertinent information. The groups are then recited. Groups are usually either four or five digits or radio-alphabet letters. The groups are typically repeated, either by reading each group twice or by repeating the entire message as a whole.
Some stations send more than one message during a transmission. In this case, some or all of the above process is repeated, with different contents.
Finally, after all the messages have been sent, the station will sign off in some characteristic fashion. Usually, it will simply be some form of the word "end" in whatever language the station uses. Some stations, especially those thought to originate from the former Soviet Union, end with a series of zeros, e.g., "00000" "000 000"; others end with music or other sounds.
Because of the secretive nature of the messages, the cryptographic function employed by particular stations is not publicly known, except in one cases. It is assumed that most stations use a one-time pad that would make the contents of these number groups indistinguishable from randomly generated numbers or digits. In one confirmed case, West Germany did use a one-time pad for numbers transmissions.
Transmission technology
radio signals transmitted at relatively low power can travel around the world under ideal propagation conditions – which are affected by local RF noise levels, weather, season, and sunspots – and can then be best received with a properly tuned antenna and a good receiver.Although few numbers stations have been tracked down by location, the technology used to transmit the numbers has historically been clear—stock shortwave transmitters using powers from 10kW to 100kW.
Amplitude modulated transmitters with optionally–variable frequency, using class-C power output stages with plate modulation, are the workhorses of international shortwave broadcasting, including numbers stations.
Application of spectrum analysis to numbers station signals has revealed the presence of data bursts, radioteletype-modulated subcarriers, phase-shifted carriers, and other unusual transmitter modulations like polytones.
The frequently reported use of high-tech modulations like data bursts, in combination or in sequence with spoken numbers, suggests varying transmissions for differing intelligence operations.
Those receiving the signals often have to work only with available hand-held receivers, sometimes under difficult local conditions, and in all reception conditions. However, in the field low-tech spoken number transmissions continue to have advantages even in the 21st century. High-tech data-receiving equipment can be difficult to obtain and even a non-standard civilian shortwave radio can be difficult to obtain in a totalitarian state. Being caught with just a shortwave radio has a degree of plausible deniability, for example, that no spying is being conducted.