The KY-8 was one the of the NESTOR family of crypto devices strictly for vehicle mounting.  They were bulky, quite heavy and mostly unreliable, but in it's heyday, it was about the only portable SECVOX (secure voice) system available to the military for use in the field. Numerous quantities were issued to the various US forces -- Army, Air Force, Navy and Marine personnel and hauled through many a battle.
KY-8 photo courtesy NSA
Joe Martin, expands on this, "This piece of  equipment played a key role in the EC-47 missions in
Southeast Asia, of which I was a part for most of 1970-71. We (the USAFSS Morse intercept operators) passed the target call sign, location, and other relevant info to the ASA troops on the ground via KY-8 or, God forbid, by "one time" pad if the secure voice failed.  A KY-8 malfunction was not cause for abort, but nobody looked forward to having to use the pads. In 270+ missions, I recall having to do it only once.
This is the KYB-6/TSEC base unit, an assembly of TSEC/KY-8. (Photo by Ralph Simpson) 
The senior radio operator (if 2 operators only) or "Airborne Mission Supervisor" (if there was a
linguist and analyst aboard) received the KY-8 keying "paddle" as part of the leather satchel of classified stuff that was picked up prior during the  USAFSS pre-flight brief. This device was, as I recall, somewhat wider than a  man's hand, about an inch thick, and maybe 6 or 8 inches in the other dimension. It had some 30-odd (again as I recall) adjustable length serrated flat bar "fingers", maybe .25 X .125 in the cross section and protected by a [spring loaded, sliding?] "hood", that were set to the prescribed positions  for that day's encryption. The paddle fingers were inserted into a mating receptacle in the KY-8 and if all went well, we were ready for scrambled voice communication. The system took a bit of getting used to, mainly due to  the delay between microphone keying and the system 'kicking in". One also  had to be careful not to key the mic until a second or two after the other guy went silent, lest the signals somehow "clash", resulting in a "beep and  a rush" (as we called it) rather than the intended transmission.

After the Cambodian "incursion" in 1970, we also used the KY-8 to transmit target transmitter locations  to "Rustic" FAC aircraft which, on a lucky  day, would provide "real time" feedback for us - -no doubt one of the VERY few  examples of that in the entire history of SIGINT".

KY-8 manual photo courtesy E-bay
Installing the KY-8 as an interim shipboard VHF/UHF  secure voice system.

References and Credits:

1) Joe Martin A292X1(at)
6981st Scty Gp, 1967-69
6994th Scty Sqn, 1970-71
2) Ralph Simpson <ralphenator(at)>
3) Nick England <>

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Oct 9/16