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See also the use of the word “close” in literature, eg The Lord of the Rings “Gandalf is closer that ever”.
To keep it close or to hold it close meant to keep it secret.
ETA: Note that I appear to have been mistaken about the connection to ENIAC. Note that it is equally dangerous to send paraphrased messages using the same key (which is called sending messages "in depth"). This was used to crack the Lorenz ("Tunny") cipher. Interestingly Bletchley Park hadn't gotten their hands on a Lorenz machine, they cracked it based on speculation. And it lead to the development of the first tube…
My assumption about “in depth” is that it comes from the idea of giving the adversary a greater depth of material to work with. I don’t have anything to back this up.
Earlier quoted context omitted.
Or ending it with the same salute involving the name of the leader, for that matter.
Standard US cryptographic protocol during the same time period was to begin and end every message with a few random words specifically to thwart such attacks.
How is this solved in modern cryptography?
The repeating of the message is how the Allies initially broke the Geheimskreiber a much more secure encryption machine to Enigma that used XOR and rotors: https://en.wikipedia.org/wiki/Siemens_and_Halske_T52
How come this isnt a problem with modern cryptography? What did we invent?
And the revolution is: It's really nice that nowadays we have telegrams that are more safe that they were during WW2 for example even with the military infrastructure available back then...
Or maybe we did have?
I forget who told me this story, but at some point the British tried a crazy known-plaintext attack by planting handwritten notes in dead German soldiers’ pockets that contained an “important message” to be sent, and then in the following days they would attempt to decrypt enigma communications against the known plaintext.
It's called operation Mincemeat
Not that this specific quirk is covered in the novel, but a reading of Neal Stephenson's Cryptonomicon would certainly help make one understand the kind of necessary paranoia that would lead to this kind of (important!) protective measure.
I forget who told me this story, but at some point the British tried a crazy known-plaintext attack by planting handwritten notes in dead German soldiers’ pockets that contained an “important message” to be sent, and then in the following days they would attempt to decrypt enigma communications against the known plaintext.