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STU-II
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.

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. The NATO version was called STU-II/B (KY-71D).

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 normal numerical keypad, some indicator LEDs and three extra buttons at the bottom.
The HYX-71A desktop unit of the STU-II (KY-71)

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.