The aliens just didn't buy the multi-planetary expansion pack so the sim doesn't contain the communications to detect.
The British came up with public key encryption well before Diffie & Hellman but since the work was for the secret service it was all highly classified. The work belonged to the government and couldn't be patented for profit as RSA has been. Bit hard to be part of history if it's all totally hush hush.
Can't remember where I heard this, some Discovery channel programme probably, but the guy that did a lot of the work wasn't allowed to take anything out of the secure building he worked in and nor was he allowed to write anything down so alledgedly did all his mathematical work in his head. Bit hard to believe really but not that implausible.
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