Stone walks us through the various SCOTUS decisions on the 4th amendment and "privacy" going back to 1928. Modern understanding of these issues is based on the 1967 decision in Katz v. United States when the court emphasized that the 4th amendment protected people not places - which basically barred the government from wiretapping a telephone without a warrant.
But then Stone describes what happened after Katz.
But Katz immediately gave rise to the question: What is a “reasonable expectation of privacy”? Suppose X has a conversation at a dinner party at which he reveals certain information about himself to the other guests. Thereafter, the government subpoenas one of the guests and compels her to testify about what X told her at the party. Is this a “search” within the meaning of the Fourth Amendment?And then he concludes:
In a series of decisions in the years after Katz, the Supreme Court said “no.” As the Court observed in Katz, “what a person knowingly exposes to the public is not a subject of Fourth Amendment protection.” Thus, as a general matter, one has no “reasonable expectation of privacy” when one reveals information to third parties.
It is on the basis of this line of decisions that commentators have correctly stated that, under existing law, the government’s program of collecting phone-call data from phone companies does not violate the Fourth Amendment.Perhaps some will argue that the recent surge in the amount of information we make available via the internet suggests that its time to reconsider the ramifications of these SCOTUS decisions. But its clear that the government's claim that the NSA programs are constitutional based on these rulings stands on pretty firm ground.