If machines like SIGSALLY were occupying so much space like in the picture above, were the pilots' communication encrypted or somehow obscured in order to prevent the enemy from eavesdropping on the time of World War II?

I know voice cryptography only started to develop that times.

  • As you pointed out, a machine such as SIGSALLY would not have fit on any plane smaller than a B-29, and only then by dispensing with either crew or cargo, or both. Transistors were not developed at Bell Labs until sometime in the 1950's, well after the war. Jan 4, 2015 at 18:11
  • If this serves as reference, I read some wikipedia article (can't remember which) that relates that in Allied bombing raids, some planes would carry german-speaking radio operators that would try to interfere in the night fighters communications. So it sends likely that the technology was not available to either side.
    – SJuan76
    Jan 4, 2015 at 21:52
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    When you say "communication" you seem to mean only voice communication. And then you answer your own question. Jan 5, 2015 at 9:55
  • @LennartRegebro With current technology, it's trivial to encrypt voice communication: your cellphone does it all the time, for example. So isn't it reasonable to ask if voice communication was also encrypted at some point in the past? (Which, indeed, your answer addresses.) Jan 5, 2015 at 10:59
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    Yes, it's reasonable, but the question answers itself by pointing out that encryption of voice during WWII required a huge machine that could not be fitted in an airplane. I think the question can be rewritten to be a good question, but currently I don't think it is one. Jan 5, 2015 at 11:24

3 Answers 3


I think it's safe to conclude that no fighter plane radios were encrypted, due to requiring extremely bulky equipment at the time. Communication between enemy fighters was theoretically possible, since all you need to do is tune in to the enemy's frequency, but most planes could only use a very limited set of preset frequencies. Of course this does mean planes with radio operators and ground stations could tune in and capture intelligence such as the enemy's heading and squadron sizes, attempt to jam the frequency, or (in the case of navigation radars) even misdirect the enemy's target.

Even extremely simple countermeasures like frequency hopping were not possible, since these require computing technology that was simply unavailable at the time. Instead, fighters employed techniques such as radio silence, codenames and frequently-changed callsigns, which last to this day.

More specifically, in the Spitfire's case, one radio it used was the TR1133 which could only have 4 preset frequencies operated via push buttons. Fighter pilots probably had more on their plate to worry about than attempt to find the enemy's frequency.

Additional reading here: http://forum.axishistory.com/viewtopic.php?t=188945

  • The use of cryptic "call signs" and jargon, with code words for targets and meeting places would serve to make clear signals useless to the enemy as an intel source.
    – Oldcat
    Jan 5, 2015 at 22:34
  • Also, the comm. between fighters and groud-control is a very short-term one. If the Luftwaffe hears, that RAF Ground Control directs a fighter squad towards an inbound bomber formation, there is very little reaction time for the Luftwaffe to devise a reaction, and to inform their own planes. So, having no encryption isn'T that big a deal on a tactical level; strategic comm. was and is always encrypted, e.g. moving a fighter squad from base A to base B. In this case, the reaction time for the Luftwaffe is large, and they could draw some benefit from the information.
    – Dohn Joe
    Sep 12, 2019 at 8:02

Encryption can very well be done on quite small portable devices, also during WWII. The famous German Enigma machine was about as large as a type-writer, and that was one of the most advanced and complex encryption of the time. Smaller machines with simpler encryption also existed.

However, encrypted communications required a separate radio person who does nothing but send and receive in Morse-code and encrypt and decrypt the messages. So fighter pilots did not do that for obvious reasons. It also takes time to encrypt and decrypt, so it can't be used for anything urgent.

Real-time encryption of voice communications did not become really practical until the 80's, and was then done through frequency-hopping.

  • So frequency hopping with analog radios wasn't feasible?
    – jjack
    Feb 25, 2015 at 18:44
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    Well, it was feasible, But you wouldn't have been able to have many frequencies or hop very fast, and you would easily lose sync and then hear nothing. It saw some limited use by Germany in WWI, however, all you need to do is to listen to all the frequencies, so it was not very useful as encryption. Feb 26, 2015 at 11:41
  • Even nowadays, frequency hopping ideas are by far not a substantial security device. Sep 12, 2019 at 2:03
  • By itself, no, but in conjunction with others it is. Sweden used, and maybe even still uses, digital radios with frequency hopping. They are nearly impossible to listen into and also very hard to jam, unless you want to jam all communications. Sep 20, 2019 at 9:16

Speech inversion was a common mode of encryption for voice communications in WWII...but it was used exclusively for ground based (mostly land line) communication. It used many vacuum tubes and was relatively unstable...and tended to drift and lose lock. Aircraft voice communication from fighters was in the clear...although users did use codes and misleading information. Morse code was used from bombers....but it too was in the clear for the most part using standard Q codes to shorten message length and time.


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