This book, in three parts, was one of the first ever written on the subject of sigint, and not by an outsider but someone who was there from the very beginning. Someone who was an expert, so much an expert that after he wrote it shortly after ww2, the nsa bought the rights to it to keep it from being published. It was hard to find a copy for a long time, however the nsa recently decided to let it be downloaded;
Parts I and II
https://www.nsa.gov/public_info/_fil...1019080017.pdf
Part III
https://www.nsa.gov/public_info/_fil...0949080010.pdf
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