For this paper I only searched for information on the Internet. There, I already found enough references, web sites, articles, and papers, so that I did not attempt to use other media to gain more information.
Although I have always tried to double-check the information I found, I have to emphasize that some of the content of this paper may not be accurate! Both the cases of Crypto AG and the Echelon system are officially not only not commented on, but one is denied information on them. Therefore, I had to use second hand information. I found in both cases many references, but all of them refer to the same sources which I also found at first.
As I already wrote in my last paper about "Social Impact of Y2k", articles written in newspapers or any other medium need not be accurate. Even if the New York Times writes about Echelon, they have very much the same sources as I have. I got the impression that what I was able to find seemed to be credible and accurate, but that does not have to be true.