Narrow-band voice encryption system
KY-9, also known as THESEUS, was a half-duplex 1 voice encryption system
developed in the US in the late 1950s or the early 1960s. The devices were
used for High Command joint circuits and were generally owned and maintained
by the US Air Force, although some of them were installed at the US Navy and
probably at other High Commands as well. The KY-9 was approved by the U.S.
National Security Agency (NSA)
for all levels of classification  and was used until at least 1973.
The KY-9 was housed is a heavy metal safe that was usually placed in a closet
or aside a desk. Inside the safe were four compartments, or quadrants,
that housed the electronic circuits. The units in the upper two quadrants
held the card readers that would take the cryptographic key on a
Remmington Rand 2 computer punch card; one for transmission and one for
The device is operated by means of a converted standard telephone set,
that has extra buttons to initiate a secure conversation, and an integrated
push-to-talk (PTT) switch 3 in the handset.
The image on the right shows a typical KY-9 setup in an office, with
the terminal on the desk.
The safe measures 110 x 78 x 59 cm and weights no less than 256 kg.
It is powered from the AC mains.
In 1962, the price of a single init was US$ 105,000.
With the KY-9 it was not possible to recognise the voice of the person at
the other end.
With a bandwidth of just 1650 baud, the speech quality was rather low,
resulting in a 'Donald Duck' style voice that was typical for the era's
Unlike the HY-2
and the KY-3,
the KY-9 was not constructed from modular electronic building blocks
(the so-called FLYBALL modules),
but rather with discrete components such as the 2N404 transistor.
The KY-9 entered service in the late 1950s or the early 1960s and was
eventually succeeded by the HY-2 /
KG-13 combination. The last units were
phased out around 1973.
Although the KY-9 datasheet states that the KY-9 is a half-duplex device [A],
at least two of its former users seem to recall that it was full-duplex .
More on this subject below.
Remington Rand computer punch cards have the same physical size
as IBM cards, but have circular punched holes, rather than IBM's
rectangular ones. The card readers are mounted vertically on the KY-9,
whereas they are mounted horizontally on other machines.
The PTT-switch was only used on half-duplex systems, or on full-duplex
systems that were used in half-duplex mode, e.g. when using a low-quality
telephone line or satellite link.
The KY-9 was also used directly by the President of the United States (POTUS),
for direct secure communication with the Chiefs of Staff, but also with the
British Prime Minister. US President John F. Kennedy used the KY-9 for the
first time in September 1961 in a conversation with British Prime Minister
Harold Macmillan. The device was guaranteed to be 100% secure .
Apart from the transatlantic link to the UK, Kennedy was also
able to contact the following people directly:
- The Prime Minister
- U.S. Embassies
- United Nations (New York)
- Chairman, JCS
- Chief of Staff, U.S. Army
- Chief of Naval Operations
- Chief of staff, U.S. Air Force
- Director, Joint Staff
- Joint War Room
- Air Force Command Post
- Continental U.S.
- CINCSAC 1
- Joint Alternate Command Element (Fort Ritchie)
- CINCAL (Alaska)
- USCINCEUR (France)
- CINCPAC (Hawaii)
CINC = Commander in Chief.
Rumour has it that President John F. Kennedy was not impressed by the
audio quality of the KY-9 and that people had trouble understanding
him. Although it was the only solution available at
the time that could be used on long-haul (narrow-band) networks,
Kennedy eventually had the much better (wide-band) KY-3
installed for communications with some State Department officials.
Half-duplex or full-duplex
The KY-9 was initially specified as a half-duplex system. It was issued with
a modified black telephone set with a dial and several extra buttons, and a
push-to-talk (PTT) switch that was integrated in the handset. The user had to
press the PTT switch when speaking, and had to release it in order to hear the
other party. At least, this was the situation in 1962 [A]:
At least two former users however, seem to recall that the KY-9 was a
full-duplex system . It is entirely possible that this is correct and that
the KY-9 was converted into a full-duplex device at some point,
just like it was done later with the KY-3
during its development. As the two forementioned users were trained on
the machine in 1969 and 1973 respectively, it is possible that by that time
the device had been upgraded, or that two of them were used in parallel.
- TSEC/KY-9 Datasheet
CSP 6620A. Department of the Navy, September 1962.
Obtained via .
- KAM-70, KY-9 Maintenace Manual (1)
- KAM-71, KY-9 Maintenace Manual (2)
- KAM-72, KY-9 Maintenace Manual (3)
- KAM-73, KY-9 Maintenace Manual (4)
- KAO-63, Operator's Manual
- KAO-65, Operator's Manual
- KAO-66, Operator's Manual
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© Crypto Museum. Created: Thursday 12 May 2016. Last changed: Thursday, 27 October 2016 - 06:41 CET.