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.
 Caption:
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!
