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Cold War
RS-804 →
Short-range agent communications - wanted item

The CDS-501 was a spy radio set with built-in data encryption, developed in the US in the late 1970s. It was used by the CIA for Short-Range Agent Communications (SRAC) in clandestine operations during the Cold War. The device was captured in Cuba in 1983 and is believed to have been used in Central and Eastern Europe as well [1]. The satellite variant is known as RS-804.

The CDS-501 is constructed of various parts, including a 1 Watt UHF short-range line-of-sight (LoS) transceiver and a CK-42 encryption device, the same one as used with the RS-804. The set was suitable for sending messages up to 1596 characters in short burst that lasted 4 to 21 sec., depending on the length of the message [3].

The CDS-501 spy radio set is extremely small, especially considering its age, and has a modular design, allowing it to be used in a variety of configurations and concealments. The thickest part of the radio measures just 19 mm, making it very easy to be hidden behind a false bottom in a briefcase or inside a piece of furniture.

The image on the right shows a complete CDS-501 set with ancillaries, hidden behind the false bottom of a wooden toolkit. It was confiscated by the Cuban intelligence services in 1983 and was shown as propaganda in the 1988 book The CIA's War Against Cuba along with various other types of concealments and spy equipment [3].
CDS-501 inside a concealment device, as compromised by the Cubans in 1983 [3].

The transmitter operates in the UHF band and produces an output power of just 1 Watt, so it had to be in the vicinity 1 of the recipient when sending its message. In practice, this meant that the agent had to drive or walk in, say, a park nearby the American Embassy (or in the case of Cuba, the American Interests Section) and deliver his message to its Electronic Dead Letter Box (EDLB).

The diagram above illustrates how the messages were sent. At the left is the agent who delivers his message to the EDLB at the embassy. The embassy relays the message to headquarters (HQ) via a secure satellite link. New instruction for the agent are sent by HQ directly via the short-wave one-way voice link as OTP-encrypted messages, for which suitable keys were supplied separately.

In addition to the CDS-501, which was only suitable for short-range agent communications (SRAC), the Americans also used a variant that was able to communicate via the US MARISAT and FLTSATCOM satellites. This radio was known as the RS-804. Examples of both radio sets were captured by the Cubans as well as by the Russians, who codenamed it PHOBOS, or FOBOS.

  1. The CDS-501 officially had an operational range of approx. 10 km, but since it operated in the UHF band, it had to be within the line-of-sight (LoS) of the receiver. Agents were instructed to drive their car to within one mile from the US Interests Section and to ensure that they could see the roof of the building [4].

The diagram below shows the layout of the CDS-501 radio set as it is currently understood. At the left is the actual CDS-501 transceiver which has a cut-out section at the bottom left. Inside this space, the CK-42 encryption unit is installed. To the right of the transceiver is the BS-98 NiCd battery pack with built-in AC mains power supply unit (PSU) that is also used as a charger.

The two units at the far right are a mystery at present, but since they are connected to the remote socket at the top right of the PSU, it seems logical to assume that the top right module holds some kind of control unit and that the one at the bottom right is a rechargeable NiCd battery.

All controls and connections are at the top. The PSU contains a 3-position rotary switch that is used to select the required mode of operation. In the center position, the unit is switched OFF. When set to INPUT, a cryptographic key and a message can be entered on the keypad of the CK-42 crypto unit. When set to SEND the unit is ready to deliver its message at the EDLB.

  1. CDS-501

  2. BS-98
    12V battery

  3. CK-42
    Encryption device

  4. ?
    Small control unit at the top right

  5. ?
    larger unit at the bottom right

  6. unmarked
    Short antenna

Operation of the CDS-501 set was extremely simple, as indicated by the small instruction label that is fitted on some devices. It shows how to enter a message and how to send it. It is likely that it could be operated by an unskilled user after just a short instruction or training. Transcribed:

  1. Switch to INPUT 1
  2. With stylus depress INPUT key 2
  3. Enter 19 character variable 3
  4. Enter twelve ↓
  5. Enter message number and an X
  6. Enter text
  7. Enter one ≡
  8. Enter four ↓
Switch to send,1 position antenna,
depress ACTIVATE.4 After 30 seconds
depress INTERROGATE.4 Observe MSG and
ACT lamps 5 for positive indication.

  1. 3-position rotary selector on the PSU.
  2. On the keypad of the CK-42 crypto unit.
  3. This is the message key that was supplied on a separate sheet.
  4. On the transceiver (aside the antenna socket), or on the remote control unit.
  5. We assume that these indicator lamps were on the wired remote control, along with the ACTIVATE button.

Captured in Cuba
A complete and operational CDS-501 set was compromised by Cuban security services in 1983 1 and was subsequently put under scrutiny. They concluded that the set was used for short-range agent communications (SRAC) by the American CIA and its operatives and agents in Cuba.

The Cubans had noticed the presence of short UHF data bursts that appeared to originate from the vicinity of the US Interests Section in Havana [4], at the time the official representation of the US in Cuba after they had closed its embassy in 1961, two years after the Cuban Revolution.

Cuban security intercepted about 4 to 5 of such transmissions on a daily basis and reported their findings to the Russian KGB, who in turn shared it with the East-Germans and the Poles, who had noticed similar SRAC signals in the vicinity of the US Embassies in their respective countries [1].
The US Interests Section in Havana (Cuba) during the 1980s. Copyright US State Department.

The Cubans used the capture of the CDS-501 and of various other spy gadgets, in the ongoing propaganda battle between them and the US. In 1988, the set was presented to the public in the book The CIA's War Against Cuba, in which it was shown inside a wooden toolkit concealment. According to the book, Cuban Security compromised quite a few CDS-501 and RS-804 devices between 1980 and 1988, some of which had been hidden by US operatives in secret caches.

  1. According to [3], the Cubans were aware of the CDS-501 radio set as early as 1980, when the first one was brought into the country. It is unclear however when the first set was compromised. In any case, they had one in their possession by 1983, when they informed the Russian KGB about it's specifications [1].

The same (or similar) radio set was apparantly used by the Americans in Poland, where the Polish counter-intelligence service intercepted mysterious UHF burst transmissions on 348.56 MHz that originated from several places in the vicinity of the US Embassy. They assumed that these were SRAC signals, and an operation was initiated to capture the source of these illicit transmissions. Although a lot of money was invested and a wealth of new equipment was bought and installed, the new political reality that had emerged by the late 1980s, made a capture unnecessary [1].

Modular design
The CDS-501 has a modular construction, which allows the radio, or parts of it, to be used in a variety of configurations and concealments. As the thickest part measures only 19 mm, it is ideal for concealment inside a briefcase or a piece of furniture. Note that some parts are also used with the satellite version of the set, the RS-804. The modules are described in more detail below.

The basic modules of the CDS-501. Copyright Crypto Museum 2015.

1. Transceiver   CDS-501
The transceiver is L-shaped and measures approx. 100 x 188 mm. It has two sockets for connection to the other modules: one in the cut-out section at the bottom left that accepts the CK-42 crypto unit, and one at the top right that connects it to the battery pack/PSU/charger. Furthermore it has an antenna socket at the top. The device is controlled from the PSU.

The transmitter operates in the 300 MHz UHF band and delivers an RF output power of 1W, theoretically suitable for an operational range up to 10 km. In pracice however, the transmitter had to be in the line-of-sight (LoS) of the receiver, due to the high frequency that was used.

In Cuba, for example, agents were instructed to drive their car to within one mile from the US Interests Section in Havana (the location of the Electronic Dead Leter Box), and to ensure that they could see the roof of the building at all times when making the transmission [3].

2. Encryption device   CK-42
Apart from the remote interface, the crypto unit is the smallest part of the set. It measures 100 x 68 x 18 mm and weights just 160 grams. It is fitted inside the L-shaped cutout of the transceiver and connects to it via a large socket at the top. It can be detached easily, allowing it to be loaded with data externally. The CK-42 is labelled as a Confidential COMSEC Controlled Item.

The CK-42 allows messages with a length of up to 1579 characters to be stored in its internal memory, encrypted with a 19 character encryption key that was supplied on a separate key sheet. It is likely that a new key was used for each message. The crypto unit also controls the transmission of the messages, which are as sent as high-speed bursts that last 4 to 21 seconds.

Messages are entered on the small keypad on the unit's front panel. It consists of 32 buttons, arranged as 4 rows by 8 columns, plus a separate INPUT button at the top. Because of the small size of the device, the buttons can only be operated with a sharp object like a stylus.

The encryption algorithm that is used by the CK-42 is currently unknown. The same crypto unit was also used with the RS-804 radio set radio set, that was used by the CIA for communication via MARISAT and FLTSATCOM satellites.

 More information
The small CK-42 encryption unit

3. Power supply unit   BS-98
The exact specifications of the power supply unit (PSU) of this radio set are currently unknown, but it is believed to be similar to the battery/PSU that was supplied with the RS-804 radio set. Any additonal information is most welcome.

 Description of the PSU of the RS-804

Known SCRAC equipment
The following US-made spy radio sets for short-range agent communications (SRAC) are currently known:

  • CDS-501
  • CDR-701 (RX only)
  • RT-519
  • RT-719
  1. Jan Bury, Project Kalina: The Lotos Operation Conundrum
    Cryptologia 36:2, April 2012, pp. 119-128.

  2. Luis Adrián Betancourt, Fidel en la Mira - Testimonio de un Acoso
    2010, ISBN 978-84-96797-35-2.

  3. José Luis Morera et al., The CIA's War Against Cuba
    Havana, 1988. Published by the Cuban National Information Service
    in cooperation with the Ministry of the Interior of the Republic of Cuba.

  4. Wikipedia, United States Interests Section in Havana
    Retrieved September 2015.
Further information
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Crypto Museum. Created: Saturday 05 September 2015. Last changed: Wednesday, 02 September 2020 - 14:43 CET.
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