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 .
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 .
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.
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 .
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
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.
- Switch to INPUT 1
- With stylus depress INPUT key 2
- Enter 19 character variable 3
- Enter twelve ↓
- Enter message number and an X
- Enter text
- Enter one ≡
- 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.
3-position rotary selector on the PSU.
On the keypad of the CK-42 crypto unit.
This is the message key that was supplied on a separate sheet.
On the transceiver (aside the antenna socket), or on the remote control unit.
We assume that these indicator lamps were on the wired remote control,
along with the ACTIVATE button.
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 , at the time the official representation of the US in Cuba
after they had closed its embassy in 1961, two years after the Cuban
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 .
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
devices between 1980 and 1988, some of which had been hidden
by US operatives in secret caches.
According to , 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 .
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 .
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 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
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 .
2. Encryption device
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
3. Power supply unit
The following US-made spy radio sets for
short-range agent communications (SRAC) are currently known:
- CDR-701 (RX only)
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© Crypto Museum. Created: Saturday 05 September 2015. Last changed: Wednesday, 02 September 2020 - 15:43 CET.