The KY-28 Speech Security System worked just as it was supposed to do.  It provided a seamless interface for audio signals produced by the radio to be encrypted and decrypted for communications security.  But with that being said, it was a failed project. 
 Designed early in the Vietnam War for use in fighter aircraft, ships, ground installations, troops in the field, and vehicles, more than $110 million was spent to develop and field KY-28s, and over 2,200 were manufactured and installed.  It interfaced with the KY-38 man pack used by ground forces and KY-8 vehicular unit.  It used a physical key (the KYK-38 key) which was used to set the key code.  It then could communicate with any other unit keyed to the same code.  It had a self-protection feature that if it registered a large impact the key would reset so that if the aircraft crashed, no one could recover the key code.
The problem with the KY-28 was that the troops and pilots refused to use them because the lag between transmission and receipt of the message was considered a dangerous distraction during combat. The result was that US air-to-air and air-to-ground communications remained unencrypted and vulnerable to enemy exploitation throughout the war.
There only external difference to the F-4B when the KY-28 was installed is that Door 629 was added to the right forward fuselage. 

KY-28 Control Panel location
in Fwd F-4B cockpit
(click on picture to enlarge)
1 - KY-28 Control Panel in FWD Cockpit

1 - Door 629 for KY-28 Speech Security System
(click on picture to enlarge)

Revision History:
  •  17 MAR 2014 - Original Post
  • Artwork by Kim Simmelink
  • NAVAIR 01-245FDB-2-1.2
  • An Analysis of the Systemic Security Weaknesses of the U.S. Navy Fleet Broadcast System, by Laura Heath, MAJOR, USA