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KY-68   DSVT
Digital Subscriber Voice Terminal

KY-68 was a ruggedized Digital Subscriber Voice Terminal (DSVT), developed in the late 1970s by the US National Security Agency (NSA) and manufactured by various suppliers, including ITT, RCA and the NSA. It has built-in encryption that uses the secret SAVILLE algorithm. A non-ruggedized variant is known as KY-78. The KY-68 was introduced in the US Army and Navy in 1992, and was in use until at least 2008. Known under various NSN numbers, including 5810-01-082-8404.

The image on the right shows a typical KY-68 unit, which is significantly larger than most ordinary phones. The reason for this is the rather large digital circuitry needed for digital communication and voice encryption, which is housed in the bottom section of the terminal. The unit shown here has been demilitarized.

The KY-68 was designed for secure voice communication in combination with a civil or military switch, using a data rate of 16 or 32 kbit/s. The unit can also be used in combination with Digital Non-secure Voice Terminals (DNVT).
KY-68 digital subscriber voice terminal (DSVT)

In the latter case, a warning tone indicates a non-secure connection. The tone is issued by the local switch and stops when the user at the other end picks up the handset. The KY-68 can also be used for secure communication between two identical sets that are connected back-to-back.

The H-350/U handset is placed in a cradle on the left half of the KY-68. It connects to the unit by means of an U-283 connector (6-pin version of the U-229). The handset has a push-to-talk switch (PTT) that is only used when the unit is operated in half-duplex mode (e.g. over radio).

The hook-switch below the handset (visible in the image on the right) can be locked by pushing it down and rotating it clockwise. This should be done when the unit is used in combination with a headset. The US Navy is known to have used the KY-68 over EHF and SHF radio links.
Pressing the push-to-talk switch on the handset

The price of the KY-68 in 1992 was US$4000. The accompanying HYP-71 power supply unit (PSU) had a price tag of US$1075 [1]. The KY-68 is currently being replaced by the Secure Terminal Equipment STE, which is backwards compatible with the earlier STU-III but not with the KY-68.

A light-weight version of the KY-68, called the KY-78, is available for office use. Except for the ruggedized case it is identical to the KY-68. According to some (unconfirmed) reports [3], the KY-78 (and therefore probably the KY-68 as well) was compromised in the early 1990s. It was nevertheless approved for SECRET-classified information. The KY-68 was still in use in 2008 [2].

KY-68 digital subscriber voice terminal (DSVT) Pressing the push-to-talk switch on the handset Hook lock switch Telephone key pad Control Panel Placing the handset in the cradle KY-68 with the handset off-hook Side view of the KY-68
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KY-68 digital subscriber voice terminal (DSVT)
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Pressing the push-to-talk switch on the handset
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Hook lock switch
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Telephone key pad
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Control Panel
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Placing the handset in the cradle
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KY-68 with the handset off-hook
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Side view of the KY-68

Technical description
Speech is digitized first using Continuous Variable Slope Delta modulation (CVSD) with a data rate that is selectable between 16 and 32 kbit/s. This allows connection to virtually any military digital switch, using digital (non-secure) voice data. In this mode, it is compatible with the TA-1042 digital non-secure voice terminal (DNVT). For signaling, it uses conditioned diphase modulation with 8-bit cyclic permuted code words. This was also used with the Philips Spendex 50 (below).

Like the Spendex 50, the KY-68 uses the GCHQ/NSA-developed Type-1 SAVILLE encryption protocol for secure voice communication, making it a member of the VINSON family of voice encryption devices. Other devices that are VINSON-compatible are:

VINSON is also used with modern SINCGARS military radios. Furthermore it is embedded into a number of multi-algorithm COMSEC modules, such as L3 Communications' unityCP custom chip and REDCOM's HDX-C encryption module. Such COMSEC modules are used as building blocks for modern cryptographic equipment.

Connections at the rear
All connections are at the rear of the unit. The KY-68 can be used in half-duplex or full-duplex mode on virtually all military communication networks. Most networks, such as the Mobile Subscriber Equipment system (MSE), require the phone to be set to 16 kbit/s, whereas the 32 kbit/s mode is used to interface with the more advanced joint/TRI-TAC switches.

The terminal is suitable for connection to a 4-wire TRI-TAC network, just like the later STE Tactical phone. One pair of wires is used for reception. It is connected to the red terminals (RVC) at the rear center. The other pair (black) is used for for transmission (XMT).

The phone can be powered by any voltage from 21 to 56V (DC) and weights approx. 6.3 kg (14 lbs). A suitable power supply unit, the HYP-71, can be connected to the socket marked 'PWR', located below the red line-terminals. A separate ground terminal is available on the right (GND).
Connections at rear panel of the KY-68

The KY-68 is also suitable for data transmission. For this, a 55-pin Digital Data Port (DDP) is available at the rear left. This connector is identical to the DDP on the TA-1042 Non-Secure Voice Terminal (NSVT). It allows connection of various digital devices, such as the Maneuver Control System (MCS) and the tactical fascimile (AN/UXC-7). Pin-out of this connector is available here...

The KY-68 is housed in a sturdy light-weight die-cast aluminium case. Unfortunately, the unit featured on this page, has been demilitarized. All of its contents have been removed (not just the crypto board), leaving the lower frame empty. In total, the unit contained 6 circuit boards.

The tamper switches are still in place and are clearly visible in the images above. One switch is located behind the front panel. When the front panel is removed, the switch ensures the destruction of the crypto variables. A similar switch is located towards the rear. It destroys the crypto keys when the top section of the phone is removed.

Rear panel with connections Connections at rear panel of the KY-68 Loading the keys Data connector Power connector (PSU) The empty lower part of the case shows where once the PCBs were located. Tamper switch at front panel Tamper switch at the top
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Rear panel with connections
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Connections at rear panel of the KY-68
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Loading the keys
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Data connector
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Power connector (PSU)
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The empty lower part of the case shows where once the PCBs were located.
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Tamper switch at front panel
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Tamper switch at the top

  • Static (wire)
  • MSRT (mobile)
  • Stand-alone (static with RT-1539)
Loading the crypto keys
Cryptographic variables (keys) are loaded into the KY-68 by means of an Electronic Transfer Device (ETD), such as the KYK-13 or the AN/CYZ-10. Such ETDs are commonly called key-fill devices or key fillers. The key filler is connected to the FILL connector on top of the unit.

The key filler can be inserted directly to the FILL connector. It can also be connected by means of a short 6-way key-filler cable, such as the one shown here. A total of three key variables can be loaded into the KY-68. The U and X variables are mandatory. The Function Selector should be set to the LDU and LDX positions respectively. The exact initialization procedure is described below.

Optionally, an extra S-variable (SVAR) can be loaded. It provides extra security between a selected group of users on a per-call basis, or in point-to-point mode. 'S' means 'Secure'.
Loading the keys

A 6.5V mercuric oxide battery (BA-1372/U or equivalent) is used to retain the crypto-variables in memory when the unit is disconnected from its primary power source. The battery is installed behind a small lid in the top of the unit, to the right of the FILL connector.

Key FILL connector Key filler Loading the keys Loading the keys Backup battery compartment Replacing the backup battery ZEROIZING the key variables Activating key loading
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Key FILL connector
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Key filler
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Loading the keys
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Loading the keys
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Backup battery compartment
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Replacing the backup battery
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ZEROIZING the key variables
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Activating key loading

Initialization of the KY-68
  1. ZEROIZE the unit by putting the VAR STOR switch in the ZERO position.
  2. Put the VAT STOR switch back in the center position.
  3. Ensure that the handset is 'on hook' or that the hook switch is locked.
  5. Connect a key filler (e.g. KYK-13) to the FILL connector.
  6. Set the KYK-13 to 'ON'. Do not press the INITIATE button on the KYK-13.
  7. Set the FUNCTION SELECTOR on the KY-68 to LDU.
  8. Select approriate KYK-13 channel (compartment) for U-variable. Ring/Busy and NSW indicator should be lit.
  9. Move the VAR STOR switch to LOAD (and hold it in position). Parity tone will be sounded.
  10. Release VAR STOR switch. Tone will be sounded to indicate U-variable is loaded.
  11. If a 10-second tone is heard (or no tone at all), variable is not properly loaded. Repeat the steps above.
  12. Load X-variable (using LDX).
  13. Load SVAR (optional).
All controls of the KY-68 are accessible from the operator's position. They are listed below:
  1. Keypad
  2. Function Selector (FUNCTION SEL)
  3. Variable Storage switch (VAR STOR)
  4. Cradle Hook Switch
  5. Push-to-talk switch (PTT) on handset
  6. Ring volume
  7. Audio volume
KY-68 controls
The KY-68 has a full numerical keypad with extra features, similar to other (digital phones). Apart from the usual 0-9 keys, it has key for R (Red) and C (white). The four red keys on the right are used for priority override (precedence):

  • C
    Conference, end-of-conference, end of dial.
  • R
    Used in certain NET-radio interface calls.
  • FO
    Flash Override
  • F
  • I
  • P
Function Selector
  • DSBL
    Disables all operations. Gives master reset to circuits (storage position).
  • LDU
    Load U-variable
  • LDX
    Load X-variable
  • OP
    Operational mode (normal use)
  • SVAR
    Load S-variable and permit 'S'-mode operation
Variable Storage switch
  • Center
    normal operation
  • ZERO
    ZEROIZE all crypto variables (pull out and move left, storage position)
  • LOAD
    Initiate loading of crypto variables (momentarily push right)
Cradle Hook Switch
  • Normal
    hook up/down detection
  • Locked
    push down and rotate clockwise. Used for headset operation.
  • Plain
    Pull out for plain text transmission (clear voice).
  • BUSY
    RING/BUSY indicator
  • EXT
    Extension indicator
  • NSW
    Non-Secure Warning indicator, flash: non-secure, ON: not initialized.
All connectors on the KY-68 are visible from the rear. They are used for the following:

  1. FILL
    Connection of key-filler
  2. AUD
    H-350/U handset
  3. EXT
    Digital Data Port (DDP)
  4. PWR
    External PSU (HYP-71)
  5. RCV
    Receive terminals (red)
  6. XMT
    Transmit terminals (black)
  7. GND
    Ground wire
    Connections at rear panel of the KY-68
Resemblance to the Philips Spendex 50
The KY-68 shows a striking resemblance to the Spendex 50 military crypto phone that was developed around the same time by Philips Usfa, for the ZODIAC integrated communications system used by the Dutch Army. Most of the controls are in the same position and the technical specifications are nearly identical. Nevertheless, the Spendex 50 offers several advantages.

The image on the right shows a typical Spendex 50 unit as it was used by the Dutch Army. Click the image for more information. Additional features on the Spendex are a red LED display, a Crypto Ignition Key (CIK), a protected ZERIOSE button and the ability to enter a key manually. It is extremely well protected against EMP strikes.

Furthermore, the Spendex 50 uses a patented system for storing distributed key-pairs, called Key-Cube key. Like with the KY-68, the key variables are loaded using a standard key transfer device such as the KYK-13 (or similar).
Philips Spendex 50 secure phone. Click for more information.

The Spendex 50 was known within the Dutch Army as Digitaal Beveiligd Telefoontoestel (DBT). It was introduced in the early 1980s and was used until the early 2000s, when ZODIAC was phased out in favour of the new TITAAN system. The Spendex 50 was approved for use by NATO.

  • Power
    21V — 56V DC
  • Auxiliary power
    Via HYP-71 power supply unit (optional)
  • Backup
    6.5V BA-1372/U mercuric oxid battery (NSN 6135-00-801-3493)
  • Mode
    full-duplex, half-duplex
  • Operation
    Via compatible digital switch, or back-to-back
  • Modulation
    Continuous Variable Slope Delta Modulation (CVSD)
  • Encryption
  • Key loading
    KOI-18, KYK-13, KYK-15, KYX-15A, CYZ-10
  • Fill cable
    ON512424 (NSN 5810-01-066-7587)
  • Test set
    TSEC/ST-34 Intermediate Level Test Set (ILTS)
NSN designators
NSN Name Registered Cancelled Manufacturer
5810-01-082-8404 KY-68 12 Sep 1979 14 Dec 2017 NSA
5895-01-250-2417 KY-68 26 Feb 1987 30 Apr 2010 ITT
? KY-68 ? ? RCA
  1. TM 11-5810-329-10
    Operator's Manual KY-68 and HYP-71.

  2. TM 11-5810-329-23
    Organizational and DS Maintenance for KY-68 and HYP-71.

  3. TM 11-5810-329-24P
    Organizational DS and GS Repair parts and Special Tools List KY-68 and HYP-71.

  4. KAO 193A/TSEC
    Guidelines for the Use and Operation of TRI-TAC COMSET Equipment (Confidential).

  5. KAM 403/TSEC
    KY-68 Limited Maintenance Manual (Confidential).

  6. KAM 404/TSEC
    KY-68 Full Maintenance Manual (Secret).
  1. Department of the Navy, Marine Corps Order 2040.14
    Materiel Fielding Plan (MFP) for the DSVT KY-68.
    MCO 2040.14, C2CT, 7 January 2008.

  2. Department of the Navy, NACMC Directive 3500.56
    Communications Training and Readiness Manual (COMM T&R Manual).
    NAVCM 3500.56, C 469, 24 October 2008. Approved for public release.

  3. Wikipedia, KY-68
    Retrieved December 2011.

  4. The Signal Leader's Guide - Field Manual 11-43
    Chapter 3-4, Subscriber Terminal Equipment. DSTV, TSEC/KY-68.

  5. LogiQuest, NSN registration for 5810-01-082-8404
    Retrieved February 2019.
Further information
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Crypto Museum. Created: Wednesday 07 December 2011. Last changed: Wednesday, 20 February 2019 - 08:39 CET.
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