You will find the quote in this book that predates Nazi Germany
Not merely was my own mail opened, but the mail of all my relatives and friends—people residing in places as far apart as California and Florida. I recall the bland smile of a government official to whom I complained about this matter: “If you have nothing to hide you have nothing to fear.”
He misattributes that quote
https://www.gutenberg.org/ebooks/1558
You will find the quote in this book that predates Nazi Germany
So the quote was about the American secret service?
Yes