The KY-171 Audio Frequency Coder was used for the NASA Project Mercury (1958-1963) to encode radio traffic between ground stations, as well as between ground stations and the space capsule. It was used with the T-560B Radio Transmitter to send encoded telemetry data. The R-669A Radio Receiver was paired with a KY-172 Audio Frequency Decoder and the C-1669 Coder Control to decipher the messages. All of these devices were part of the AN/FRW-2 Radio Set.

KY-171 Audio Frequency Coder

Now… I have been looking around the internet for weeks now, but I can not seem to find any specification or paper describing the encryption/decryption algorithms/techniques used. The only thing I've been able to dig up is the schematic, which is printed on the inside of the bottom plate of the coder itself.

schematic printed on the inside of the bottom plate of the coder

Can anyone confirm if any written documentation related to the used crypto is available to the general public at all? If it is, where can I find relevant papers describing the cryptographic algorithm(s) and/or techniques used within KY-171 and KY-172?


1 Answer 1


To my knowledge, Mercury Project used keyed CWI (Continuous Wave Interrupted) FM (frequency modulated) system, or FMCWI, capable of receiving Morse code over the UHF (Ultra High Frequency) and voice over HF (High Frequency) channels. This would be a completely analogue system, so we can't really speak of cryptographic algorithms. That doesn't mean comms were not tamper-resistant, though.

                                                 AM and FM carrier waves

                   Sinusoidal FM Modulation identifies range by measuring the amount of spectrum spread produced by
                 propagation delay. AM is not used with FMCW. (Source: Wikipedia on Modulated Continuous-wave radar).

It would come with anti-jamming capability in the form of multiple frequency modulated narrow-band tuned filter carrier waves, and since its operation is similar to Continuous Wave (CW) radars, with interrupted carrier system (think of it as half-duplex, requiring only a single antenna in contrast to non-interrupted CW) in place, the system would have ranging capability, so any attempt at transmitting false commands from a different location would, to my understanding, likely be detected and the operator could switch comms device / channel.

Two coder units were used, the master and the standby unit, both used simultaneously during system operation, with a monitoring device detecting failure and causing switchover to the remaining coder in case of a detected lack of modulation.

             FMCW Intruder Detection

         Doppler signature of Frequency Modulated Continuous Wave (FMCW) radars (Source: FLIR Surveillance Technologies)

More additional information on the comms system can be read in the INTRODUCTION TO PROJECT MERCURY AND SITE HANDBOOK, National Aeronautics and Space Administration (NASA) document from September, 1960 (PDF).

  • $\begingroup$ The transmission mode is analog, but the content is digital -- Morse Code. I only skimmed through the linked handbook, I didn't see too much detail about the secured comms. That said, if you encrypt a message and then convert it to Morse, you can pass a digitally encrypted message over the analog channel. $\endgroup$
    – WPrecht
    Nov 11, 2013 at 19:24
  • 1
    $\begingroup$ @WPrecht Sure, but the question was about the hardware. OP is well versed in IT security, cryptography, e.t.c. so he'll realize that. ;) $\endgroup$
    – TildalWave
    Nov 11, 2013 at 19:43

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