The KY-3 was a secure voice encryption/decryption (ciphony) device used for communicating sensitive information by voice. It was used by the U.S. military and almost every other branch of the United States government from the mid 1960's through the late 1980's.

The KY-3 used Pulse Code Modulation (PCM) to digitize the analog voice signal and then XOR'd the encoded digital voice stream with its internally generated digital encryption key-stream. This resulted in a 50 kb/s (wideband) encrypted digital stream that could be sent over non-secure wideband channels. To decrypt the encrypted digital stream, it was XOR'd with its original digital encryption key-steam (produced by an auto-synchronized internal key generator) to yield an unencrypted PCM digital stream that was subsequently demodulated into an analog voice signal.

To keep things interesting for an unintended party intercepting and attempting to decrypt a KY-3 digital stream, the lower-value binary digits of the PCM signal were randomized using an internal electronic noise signal generator. This low-level noise had little effect on the quality of the decrypted and demodulated voice signal, but it served well in adding a purely random attribute to the overall digital stream issued by the KY-3; thus, making it more difficult to analyze the encrypted digital stream for key generator and synchronization data patterns. Using this technique produced very secure voice communication with good speech quality -- much like the quality experienced with today's digital voice telephony services.

The complex digital circuitry of the KY-3 was built entirely with discrete transistor components in multi-colored potted modules, mounted on plug-in circuit boards -- precursors to today's small scale integrated circuits. The circuit boards were plugged into inter-wired sockets contained within three drawers. Essentially, one draw contained the transmission circuitry, another the receiver circuitry, and the third -- bottom -- draw contained the line interface, clock and power supply circuitry. This was all placed in a safe-like, table-height container with safe-like door and combination lock.

The KY-3's digital key-stream generator was variably configured (i.e., wired) via hole-punched Hollerith cards -- known as key-cards -- placed into electrically wired, through-hole point-contact Hollerith card readers mounted on the front of the transmitter and receiver drawers. The user was interconnected to the KY-3 via a remote desk set - either KYX-9, KYX-9A, or KYX-10. The KYX-9/9A desk sets resembled standard, black, rotary telephones, with the addition of extra buttons and lights along the bottom edge. The KYX-10 was a larger multi-line unit, and was available in colors other than black.