[Cryptography] Imitation Game: Can Enigma/Tunney be Fixed?

[Jon Callas]

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On Jan 8, 2015, at 1:35 PM, Ryan Carboni <ryacko at gmail.com> wrote:

> If we're doing alternate history and second guessing the decisions of the Germans while they had limited resources.... why not use a Feistel cipher with six letter blocks in ECB mode? Such a machine would only weigh 50 kg... not a major problem?

That's about 45kg too heavy.

But in any event, you'd be scrod. Go read "The man who broke Napolean's codes" and see how one lone guy did it against Napoleon a century and a half before that.

ECB is Electronic *Code* *Book*. It doesn't matter how good the algorithm that produced the code book is, you just build the code book. You start doing statistics on the ciphertexts and look for duplicates. You make the guess of which messages are weather reports, and so on, just like they did against the stream cipher. Then you start piecing it together.

Also go look at the Venona history, as well as the fact that the three-byte "IV" in WEP led to a 24-bit attack against RC4 no matter what the key size was.

It's probably easier to break than Enigma.

On the other hand, at 45kg, it's so heavy that you wouldn't have as many machines and thus less traffic and less chance at playing statistics. Thus, ironically, the heavier weight would have been a problem for the cryptanalyst.

On the other, other hand, that would have also meant more plaintext traffic, too, and that's much easier for the cryptanalyst. Most people can break XOR-with-zero with pad and paper.

	Jon



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