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Speech encryption device - wanted item

TSEC/KY-38 is a half-duplex manpack voice encryption device, introduced around 1967 by the US National Security Agency (NSA) . The device is part of the NESTOR family of speech encryption devices and was heavily used during the Vietnam War. 1 It is compatible with the KY-8 vehicular unit and the KY-28 airborne unit. The KY-38 was succeeded in 1975 by the KY-57 (VINSON) [3].

The image on the right shows a US soldier with an PRC-77 manpack radio on his back, during the Vietnam War in October 1969. Below the radio set is the KY-38 voice encryption unit, which is about the same size as the radio itself.

The handset is connected to the KY-38, while the KY-38 is connected to the 14-pin socket of the radio, via a long thick cable. This cable pro­vides the KY-38 with power, whilst connec­ting the input and output of the device directly to the transmitter and receiver circuits, there­by effec­tively bypassing the audio circuits of the radio.

The radio provided excellent security but was nevertheless hated by its operators. The audio quality was not very good and the total weight was close to 25 kg. Furthermore, there is a 600 ms synchronisation delay each time the push-to-talk switch is pressed. In addition, the device was running hot in the burning Vietnamese sun, prompting technicians to cover it with wet cloths and drill unauthorised holes in the enclosure to provide sufficient cooling and avoid break-down.

The KY-38 was introduced in 1967, together with the airborne KY-28 and shortly after NESTOR member KY-8. They were first used in the Vietnam War and were initially only available in small numbers. In 1974, the KY-38 was succeeded by the KY-57 (VINSON). The last NESTOR units were decomissioned in the early 1980s. In total, approx. 30,000 NESTOR devices were produced.

  1. The Vietnam War lasted from 1 November 1955 to 30 April 1975.  Wikipedia

Photograph of soldier with
PRC-77 and KY-38
The photograph above was obtained via Wiki­pedia, where it is attributed to the NSA [4]. Note how­ever, that the image as supplied by the NSA is mirrored horizontally, showing the controls and connections of both devices in the wrong position. The above version has been corrected.

The image above appears to be a cut-down version of this photograph, in which the entire antenna and the soldier's feet are fully visible. According to the caption at the back of the photograph, it was made in Vietnam on 16 October 1969 by SP4 Timothy F. Fease. It was released for publication on 30 january 1070.

Interestingly, the cover of the technical manual of the KY-57 — the successor of the KY-38 — shows a drawing of a soldier in the exact same position, but with a KY-57 strapped to the harness instead of the KY-38.

PRC-77 radio
The AN/PRC-77 radio was in fact a PRC-25 that had been given a solid state RF power amplifier, and was modified for the connection of digital voice encryption equipment, by adding a so-called X-MODE to the circuitry.

When in X-Mode, the filters in the audio path of transmitter and receiver are bypassed so that the full bandwidth of the set can be used.

 More about the PRC-77


  • Device
    Wideband voice encryption device
  • Purpose
    Secure military communications
  • Model
  • Year
  • Family
  • Country
  • Successor
  • Encryption
  • Modulation
  1. Brooke Clarke, AN/KY-38 Secure Voice System
    Retrieved November 2016.

  2. SP4 Timothy F. Fease, Image of soldier with AN/PRC-77 and TSEC/KY-38
    16 October 1969. 53 Sig Bn. 3791-30/AGA69. 1

  3. Wikipedia, NESTOR (encryption)
    Mirrored image of soldier with PRC-77/KY-38.
    Retrieved November 2016.

  4. Wikipedia, AN/PRC-77 Portable Transceiver
    Retrieved September 2019.
  1. Declassified by USAPA, 30 January 1970.

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Crypto Museum. Created: Saturday 01 October 2016. Last changed: Wednesday, 01 May 2024 - 06:54 CET.
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