KY-28 (Nestor)

The KY-28 was an analog, voice encryption device based on transistor circuitry and was the shipboard/airborne member of the NESTOR family of equipment.

With reference to the photo, the KYK-38 (the black box on top of the KY-28) was used to input the key set. Surrounded by a clamshell cover, it contained rows of pins corresponding to the rows of small holes on the front of the KY-28. It was keyed  through a small coverplate, that when opened, exposed a number of holes.  A key charging device was used that was comprised of slide stops on a series of rods and a cam mechanisim to force the rods out to the distance set by the slide stops against spring pressure.  The rods would penetrate the holes and set permuter contacts according to the depth of penetration. It took a lot of pressure to load a key set. A zeroize pin was activated by closing the cover, such that opening the cover again would zeroize the permuter settings, which would also render the equipment incapable of passing an alarm check and passing to operating mode.  For aircraft operation, the policy was to wire the door shut using safety wire before the aircraft was cleared for flight.

The charging device was set according to a distributed and classified key list. The KY-28 equipment was owned by individual services and held the classification of the keys when loaded.

ky28.jpgCaption: The black block sitting on top of the KY-28 is the KYK-38 and it plugs into the front of the unit. This device is on display at the MARCOM Museum in Halifax. (Photo by Jerry Proc)

The device was also fitted with a internal shock sensor. If the aircraft crashed, the resulting 'G' forces would trip this sensor thus resetting the key.  This measure was implemented in order to prevent a keyed up device from falling into enemy hands. Sometimes the sensor tripped as a result of a hard landing. As a further security precaution, the KYK-38 key plug was left on the ground. The KYK-28 had two lights as well as a plain/cipher switch. A green light indicated cipher mode and a red light was used for plain  mode. If the unit was zeroed, the green would not light up even if the cipher/plain switch was switched to cipher. It would default to plain mode only.

Control of the KY-28 was facilitated by mounting a KYK-28 control panel in the cockpit of the aircraft. The pilot's microphone was plugged into the KYK-28 control assembly and the encrypted audio output as well the transmit keying line was wired directly into the  audio and control inputs of the transmitter of transceiver. Voice quality and recognition using this crypto device was awful. Even under optimal conditions, every one sounded like Donald Duck. The only application where I seen it used was in FM radio voice communications. The KY-28 system was replaced in the 1980's by a newer family of equipment that used a digital encryption scheme.

There were several connectors below the charging port, for audio, digital, power and remote control.  Cases were painted green or NSA gray for tactical land vehicle use or airborne service respectively.  Speech was encoded usiong the CVSD method and ran at 20 some kilobaud to match the bandwidth of a particular radio.

Operating the set was not without problems. Initialization delays were caused by two factors: crypto synchronization delays and digital transmission synchronization delays.

Crypto sync delays could have been alleviated with cheaper digital storage (to MILSPEC) and/or by using other design principals for coming up with message indicators (initial vectors), the limiting factor being digital storage.  During the era, fast training MODEMs were a bit of a rarity, covered I believe by a patent held by Bell Labs.  Some of the delay might have been due to digital synchronization which could be overcome more readily today.  Generally the push to talk switch had to be pressed, then wait for the synchronization tones.

Slow synching equipment was better suited to full duplex installations not requiring frequent
resync.  The implemented message indicators were slow, long but relatively error immune.  Trying to speed up the message indicator would call for error correction, which was rare and expensive during the era.  Once  synchronized, an isosynchronous system using CVSD is not particularly susceptible to transmission errors, which causes only momemtary distortion as long as no clock event errors occur.  Speeding up the message indicator would have required faster hardware and might have been technically  unfeasible or prohibitively expensive.

Field level maintenance for the KY-28 was restricted to remove and replace procedures.  Troubleshooting the internals would consist of the subsitiution of suspect subassemblies with known good items according to a troubleshooting chart.

Most of the KY-28's available today were used in Vietnam.  Their introduction to service was predicated on the fact that the North Vietnamese were intercepting plaintext radio transmissions and exploiting the resulting gathered intelligence, as was done against the U.S./Allies by the Germans and Japanese in WWII.

However, using the KY-28 in an aircraft when being fired at was impracticable because of the crypto
synchronization time required. Pilots being shot at were not about to wait for the crypto to sync up so they communicated in the clear! The ground troops would do the same!

C-8157/ARC control head. (Photo via e-Bay:

The  was used with the KY-28 in aircraft installations.  There was at least one other type of smaller control head which was also used with the KY-28 in aircraft with limited cockpit panel installation space (see below).  When aircraft were retrofitted with the new KY-58 VINSON systems, in some cases the older KY-28 control head was removed and the KY-58 was directly installed within the cockpit radio panel since the size of the KY-58 is approximately 6 x 6 x 8 inches deep.  The KY-58 could also be remotely mounted in the same mounting tray as the old KY-28 through the use of a remote control adapter.  If the KY-58 was remote mounted, the KY-58 could be configured to utilize the old KY-28 control head or the newer KY-58 control head.  The advantage of using the new KY-58 remote control head is that all of the functions of the KY-58 could be operated from the control panel,  while the old KY-28 control head would only allow selection of the Plain or Cipher operating mode of the KY-58.

Here is another version of the KY-28 (C-8057/ARC 1968)  control head last installed on a A4 Skyhawk. It had a 1968 contract date. (Photo courtesy e-Bay).

KY-28 rear view. (Photo via E-bay)


1) David Koontz <koontzd(at)>  Newsgroup:  sci.crypto
2) Ray Rolison, ex-Crypto Tech, U.S. Army
3) George Mace  <gmace8(at)>
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April 14/07