TSEC/KY-3 Wideband Voice Encipher/Decipher

The KY-3 was a full-duplex, wideband (50 Kilobits per second), voice encipher/decipher (ciphony) device used for securely communicating 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 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 transmitter (encipher) key generator circuitry, another the receiver (decipher) key generator circuitry, and the third -- bottom -- draw contained the wire-line interface, analog-to-digital voice encoding/decoding, system clock and power supply circuitry.  This was all placed in a safe-like, table-height container with safe-like inter-door and combination lock that was behind a lever-latched outer-door, as show below.

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 several colors.

The KY-3 employed pulse code modulation and demodulation (PCM) to digitally encode and decode an analog voice signal.  For secure transmission, the PCM encoded voice signal's digital stream and timing bits were then XOR'd (i.e., modulo-2 arithmetic addition) with a pseudo-random digital key-stream, produced by the transmitter's key generator.*  This resulted in a 50 Kb/s enciphered digital stream that could be sent over four-wire, non-secure wideband channels.  For reception, the enciphered PCM digital stream was deciphered by XORing with its original digital key-steam -- produced by the receiver's key generator. To synchronize the receiver's key generator with a remote transmitter's key generator, its clocking was automatically accelerated until the transmitted timing bit-pattern was detected in the XOR'd output stream.  The deciphered PCM digital stream was then demodulated to recover the analog voice signal.
To keep things interesting for an unintended party intercepting and attempting to decipher a KY-3 enciphered digital stream, the lower-value binary digits of the PCM signal were randomized via an internal electronic noise generator.  This low-level noise had little effect on the quality of the deciphered and demodulated voice signal, but it served well in adding a continuous and purely random attribute to the enciphered digital stream issued by the KY-3; thus, rendering it difficult to analyze the enciphered digital stream for key generator and synchronization bit patterns.  This technique produced very secure voice communication with very good speech quality -- much like the quality experienced with today's digital voice telephony services.
*The KY-3's pseudo-random digital key-stream generator was essentially a high-degree Fibonacci linear feedback shift register that was variably configured and seeded (i.e., initialized) daily via hole-punched Hollerith-like cards (i.e., a Koken type pseudo-random key stream generator).  The "key-cards" were placed into electrically wired, through-hole point-contact Hollerith-like card readers mounted on the front of the transmitter and receiver drawers.