Wide-band voice encryption system
KY-3 was a secure voice encryption system,
or ciphony system,
developed by Bendix Corporation in California (USA),
around 1962 and
introduced with various branches of the US military in 1963.
The system was used by the US military and also by other branches of the
US Government, such as Congress and the State Department .
The KY-3 was intended for use on wideband radio and telephone channels,
and featured Pulse Code Modulation (PCM) for speech digitisation, resulting
in excellent voice quality, where one was able to clearly recognise the person
at the other end [A].
The KY-3 is also known as TSEC/KY-3 and by National Stock Number
The KY-3 weights 136 kg and is housed in a safe that has the size of
a medium-sized refrigerator, which can be locked just like an ordinary safe.
Inside the cabinet are three rackmount drawers in which the
electronic circuits are housed.
Each drawer can be pulled forward for servicing and
adjusting, as shown in the image on the right.
Two drawers have a card reader mounted to their front panel: one
for the transmission circuit (TX) and one for the reception circuit (RX).
The card readers were used for loading the cryptographic keys,
which were supplied on regular
Remmington Rand computer punch cards. 1
The TSEC/KY-3 is operated by means of a so-called terminal unit or
remote desk set, which is in fact a modified ordinary telephone
set with extra buttons and features. Known desk sets for the KY-3
are the KYX-9,
and KYX-10. The one shown in the image is the KYX-9.
The TSEC/KY-3 was one of the first fully transistorized devices that used
the so-called FLYBALL functional building blocks.
At its introduction in 1963, the price of a single KY-3 was US$ 18,000 [A].
More than 2500 units were built for the National Security Agency (NSA) .
President John F. Kennedy preferred the KY-3 over the far less intelligible
KY-9 and had it installed at the homes of several government
The KY-3 was succeeded in 1977 by the Secure Terminal Unit
and later by STU-II,
The last KY-3 units were taken out of service in 1994 when the final
SECORD/AUTOSEVOCOM secure voice switch was deactivated at the Pentagon .
Remington Rand computer punch cards have the same physical size
as IBM cards, but have circular punched holes, rather than IBM's
The image on the right shows a typical KYX-9A desk set that was built in 1973.
It is basically a multi-line 566MD telephone made by Western Electric, of
which the keys and the lamps
have been assigned different functions. For this,
the telephone set was slightly modified internally.
The KYX-9 is made of black plastic and has a rather heavy black plastic handset.
The unit is connected to the KY-3 by means of a
multi-wire cable with a
50-way Amphenol plug at the end.
As it has an old rotary dial,
it is not suitable for priority override on AUTOSEVOCOM networks.
US President Kennedy preferred the KY-3 over other voice encryption systems
such as the KY-9 and the
because of its much better audio quality. Rumour has it that people had trouble understanding Kennedy on narrow-band encrypted
phones, which is why several KY-3 systems were installed at the homes of some
high ranking State Department officials . In such cases the KY-3 was
usually hidden inside a closet or disguised as a piece of furniture, whilst
a KYX-9 desk set was placed on the table. The KY-3 was connected to a private
exchange via subscriber lines.
In some situations, e.g. when KY-3 was used aboard a ship, the KYX-9
desk set was replaced by an internal KYX-10 telephone set, which was built
inside the empty 'drawer' space above the safe door of the KY-3.
The image below shows a KYX-10A which was on public display at
The unit is built from standard Western Electric telephone parts and consists
of a black plastic handset, a rotary dial, a three-position MODE selector,
and six push-buttons with built-in indicator lamps. The MODE selector is used
to choose between CIPHER, PLAIN and ON-HOOK.
The six push-buttons at the right were mainly used for their indicator
lights and showed the user whether the connection was secure or not. In
case it was not secure (i.e. PLAIN), the second lamp would be lit.
The first push-button could be used as Push-To-Talk (PTT) when the set
was used in half-duplex mode (e.g. on two-wire lines).
The text below the buttons can be viewed here.
The KY-3 was usually connected to a local SY-1 switchboard, which could
accomodate 12 KY-3 units, expandable to 23 units, with the ability to
use 3 active lines (talking lines) simultaneously. In some configurations,
the KY-3 was used to secure short haul subscriber lines to the
which was in turn used on long haul narrow-band secure voice circuits [A].
The above diagram shows a common setup that involves two (or more) KY-3
units and an SY-1 switchboard, connected via a wideband subscriber line
(max. 20 miles), and a HY-2 /
KG-13 combination that converts the voice data
into a secure narrowband long distance signal.
The KY-3 was initially developed as a half-duplex system, but at some point
during the development, the Navy changed its requirements to full-duplex,
which the Bendix engineers managed to achieve.
The KY-3 was supplied in a few flavours and was also modified, improved
and enhanced several times during its operational life.
The photograph on the right was taken from the NSA's scrapbook 
and shows one of the first KY-3 units as it was used in an office
alongside a regular desk.
The safe in which the KY-3 is housed, has the same height as the
desk itself (approx. 72 cm), and the KYX-9 terminal is placed on top.
The empty 'drawer' just below the unit's desktop is clearly visible here.
It was used on some military versions to accomodate an
'internal' telephone set with handset and rotary dial (see below).
The empty 'drawer' space, just above the safe door, was provided for the
accomodation of the KYX-10 internal telephone set,
as shown in the image on the right .
The one shown here is a KYX-10A, which is build from
standard Western Electric telephone parts. It consists of a handset (here
shown on-hook), a rotary dial, a toggle switch to select between CIPHER
and PLAIN, and six push-buttons with built-in indicator lamps.
In this image the safe door is closed and a (green) warning text is present
above the lock: Access beyond this point requires two person control,
which was an extra security measure.
The stacked KY-3 unit shown here was on public display at the NCM in 2011,
and had a descriptive text
on the main door  (which is open here),
suggesting that the KY-3 had been in production between 1965 and 1967 only,
at a unit price of US$ 5,000 each. We believe this to be incorrect however,
as the naval
Cryptographic Equipment Planning and Reference Guide,
issued in September 1962 [A], already shows the KY-3 and quotes the higher
unit price of US$ 18,000.
Furthermore, it is known that US Presedent John F. Kennedy 1 used the KY-3
for confidential conversations with high ranking members of the
State Department, which means that the unit was already available in 1963.
As the first KY-3 units were replaced by the STU-I in 1977, it is likely that
the KY-3 was in production until at least the early 1970s.
John F. Kennedy was murdered on 22 November 1963.
Like its narrow-band counterparts
the KY-9 and
the HY-2 /
the KY-3 is fully transistorised
and consists of NSA-developed
FLYBALL modules: brightly coloured
LEGO-style functional building blocks, each containing discrete components
that provide a single logic function,
as indicated by its unique colour.
The image on the right shows a set of printed circuit board (PCBs)
with pink coloured FLYBALL modules, as they are present inside the
KG-13. The construction of the KY-3 is very similar.
➤ More about FLYBALL modules
- TSEC/KY-3 Datasheet
CSP 6620A. Department of the Navy, September 1962.
Obtained via .
- KAO-77, Operating Instructions KY-3
- KAM-128, Maintenance Manual KY-3
- KAM-129, Maintenance Manual KY-3
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© Crypto Museum. Created: Thursday 12 May 2016. Last changed: Tuesday, 28 September 2021 - 15:08 CET.