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TSEC/KY-71 Secure telephone
The STU-II was the second generation secure telephone
developed by the NSA in the 1980s.
It replaced the STU-I,
KY-3 and
Navajo-I.
The STU-II is also known as the KY-71 or TSEC/KY-71.
It was replaced in the 1990s by the STU-III.
For NATO it was replaced by the STU-II/B.
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The STU-II was built by ITT, using Northern Telecom as a sub-contractor.
The unit consisted of a large metal cabinet that contained the electronics
(the actual KY-71) and a telephone-style desktop unit, known as the HYX-71,
that acted as the user interface.
The image on the right shows the HYX-71 desktop unit, that was connected to
the KY-71 cabinet by means of a 9-way cable.
It's a fairly large telephone set, with a standard handset,
a normal numerical keypad, some
indicator LEDs and three extra buttons at the bottom.
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Once a call was established in CLEAR mode, the user had to press the SEC-button
to 'go secure'.
As the STU-II uses the same LPC-10 vocoder as the later
STU-III, there is always a 10 to 15 second delay
before secure mode is activated. During this stage, the keys are exchanged.
Voice data was transferred at 2400 baud, resulting in a very synthetic sound.
Although speech was relatively clear, it was impossible to recognize the speaker
at the other end. Certain versions of the STU-II were able to use 4800 baud,
but given the rather poor quality of telephone lines, 2400 baud was used in
most cases. When in half-duplex secure mode (simplex), the user had to press the
so-called Push-To-Talk switch (PTT) inside the grip of the receiver when speaking.
Key material was handled by a so-called Key Distribution Center (KDC), which
was NSA operated. Keys were transferred to the STU-II by means of a
KOI-18
(paper-tape) or KYK-13 (electronic)
key transfer device.
Approximately 10,000 STU-II units were built [2].
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Shortly after the introduction of the STU-II,
Philips Usfa
developed the Spendex 40.
It was intended for the Dutch Army and the Dutch Government. As Philips also
planned on selling the Spendex 40 to NATO, they were licensed by the
NSA to use the
SAVILLE encryption algorithm.
Spendex 40 was fully interoperable with the STU-II but was much smaller.
The entire unit was haused in a single case. Furthermore, Philips developed
its own LPC-10 vocoder based on modern DSP technology, giving the Spendex 40
better speech quality than the STU-II.
 More information
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In the early 1990s, the NSA introduced the STU-II/B.
It was built by Motorola and was intended to
replace all STU-II compatible devices, including the STU-II itself and the
Philips Spendex 40.
The STU-II/B was much smaller, but its hardware was not related to the
original STU-II in any way. Instead it was based on the later
STU-III, with the most significant difference being the
presence of a connector for the key filler at the rear.
More information
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© Copyright 2009-2012, Paul Reuvers & Marc Simons. Last changed: Mon,14 May 2012.09:44:35
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