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Can the cryptographic algorithm DES be implemented in a electromechanical machine with the technology of the early 20th century (1900-1938)?

Important Note

With technology of 1900-1938, I refer to only using something that can be orders to a professional electromechanical enginer of that epoch, giving him the blueprints (don't matter how you get the blueprints) and nothing more.

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    Why not? What would prevent it from being implemented? You can use pen and paper to implement it, or any other technology to do the computations, it does not even need to be an electromechanical machine.
    – Justme
    Dec 29 '21 at 15:05
  • I guess the main risk in the XXth century is coming under fire from Mega Man?
    – Tommy
    Dec 29 '21 at 15:13
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    Please give us a more specific year range. The title says "XXth century", but the body says "early XXth century." Also, use decimal numbers, not roman numerals.
    – DrSheldon
    Dec 29 '21 at 15:43
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    This seems to be a question about a non-existent machine which probably wouldn't be considered a computer if it had existed. Dec 29 '21 at 15:57
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    @wizzwizz4 not to mention that it's really bad practice to tweak the wording of a question just to make it legally on-topic. RC.SE isn't a court house battle, but a joint search for truth.
    – Raffzahn
    Dec 29 '21 at 23:37
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I don't see why not, in principle. The DES algorithm consists of iterative shifting, mixing, and XOR operations. There would be no fundamental barrier to constructing something like a relay-based special-purpose computer that could calculate that. Such a machine would be rather large though, in the same ballpark as the largest relay computers at Bell labs in the 1940s. (Room-sized.)

This is only speculation on my part, but I think this wasn't done historically (not necessarily with DES per se, but at all) because electromechanical rotor systems were thought secure enough, right up to the British breaking Enigma. Further, the math hadn't been developed yet. Both modern block and stream ciphers are derivative from Shannon's proof of perfect security for XOR + and a one-time-pad. That wasn't published until 1949, though Allied cryptographers were aware of it earlier during WW II.

Which brings me to SIGSALY. Not quite what you asked but you may find it interesting. During the war, the Allied Powers wanted secure radio communication for speech. The solution was to digitize it to PCM audio, feed that into a vocoder, digitally sum the vocoder output with a one-time-pad to encrypt it, and broadcast the digital signal over FSK modems on shortwave. The whole thing was then reversed on the receiving end. The encryption stage turns out to have been redundant. The Germans never figured out it was even audio data; they suspected it was some form of multiplex telegraphy or over-the-horizon radar. https://en.wikipedia.org/wiki/SIGSALY

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Can the cryptographic algorithm DES be implemented in a electromechanical machine with the technology of the early XXth century?

Of course. There is no reason against.

Except maybe the most important:

  • No Data to Encrypt

thus.

  • No Use Case

I guess it's safe to pinpoint the era asked about as up to 1940, i.e. before the war. At that time data storage was punch cards and decimal values. Neither with a need to encrypt nor the binary base to apply DES like algorithms. Thus no application in computing.

The only area with a possible use for encryption might have been communication. While we today tend to see applications like (Baudot) TTY as binary, they weren't. They were symbol based with a per-symbol serialization that may seeming like binary, but isn't, as it's not representing values. Only a sequence.

DES operates below the level of symbols on binary representation of symbols. This is only possible by translating symbols into binary sequences, manipulating (encrypting) them, followed by re-creation of symbols to be transmitted. And of course reversing the process on receiver side.

Quite contrary to today's technology, where everything is geared to pile up the most simple units (binary) units, as electronics can produce them quite easily, and connections thereof can be made almost indefinite, mechanical computing works best with as few units as possible with the least connections necessary. Making each unit more complex, like using higher order symbols like decimal or letters, is the way to make it go.

Thus any kind of useful encryption machinery should have been symbol based - which is exactly what Enigma-type machines were. In practical use they had several advantages:

  • Direct handling of symbols within symbol space (no unnecessary translation hardware)
  • Short key phrases (easy to handle for humans)
  • Fast operation (not slowing down transmission
  • Small machine size (important for mobile use)

All of that would be quite hard, if not impossible, to reach with a binary based algorithm.


In fact, Enigma-type encryption could quite well offer comparable security to DES - if applied right. It can be said that, without any disrespect to the English effort, that German hubris was the most relevant factor in code cracking.

Most important here the practical elimination of most of the codespace (the equivalent of a more than 70 bit code word was reduced to less than 20 bit), redundant messages and fishing. Most of these shortfalls were well known, but accepted. After all, Germany expected to win the war before anyone could come up with useful decoding.

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