The 1886 case of
Boyd v. United States,[1] one of the first to look closely at the fourth amendment, held that the government could not work around the amendment by subpoenaing incriminating papers directly from the person to be charged:
> But we do not find any long usage, or any contemporary construction of the Constitution, which would justify any of the acts of Congress now under consideration. As before stated, the act of 1863 was the first act in this country, and, we might say, either in this country or in England, so far as we have been able to ascertain, which authorized the search and seizure of a man's private papers, or the compulsory production of them, for the purpose of using them in evidence against him in a criminal case, or in a proceeding to enforce the forfeiture of his property.
The 1906 Supreme Court case of Hale v. Henkel addresses a corporation's fourth amendment challenge to a subpoena for the production of documents relating to the company's correspondence with third parties. The Court denied the company's fourth amendment challenge:
> We think it quite clear that the search and seizure clause of the Fourth Amendment was not intended to interfere with the power of courts to compel, through a subpoena duces tecum, the production, upon a trial in court, of documentary evidence.
The 1877 case of ex parte Jackson considered the issue of U.S. mail, and stated, in dicta, that it was protected from search by the fourth amendment:
> No law of Congress can place in the hands of officials connected with the postal service any authority to invade the secrecy of letters and such sealed packages in the mail; and all regulations adopted as to mail matter of this kind must be in subordination to the great principle embodied in the fourth amendment of the Constitution.
The 1928 case of Olmstead v. United States held that wire-tapping of telephone conversations was not protected by the fourth amendment:
> The Amendment itself shows that the search is to be of material things — the person, the house, his papers or his effects. The description of the warrant necessary to make the proceeding lawful, is that it must specify the place to be searched and the person or things to be seized.
Olmstead distinguished ex part Jackson as being based on the peculiar relationship between the fourth amendment and the postal monopoly:
> It is urged that the language of Mr. Justice Field in Ex parte Jackson, already quoted, offers an analogy to the interpretation of the Fourth Amendment in respect of wire tapping. But the analogy fails. The Fourth Amendment may have proper application to a sealed letter in the mail because of the constitutional provision for the Postoffice Department and the relations between the Government and those who pay to secure protection of their sealed letters. See Revised Statutes, §§ 3978 to 3988, whereby Congress monopolizes the carriage of letters and excludes from that business everyone else, and § 3929 which forbids any postmaster or other person to open any letter not addressed to himself.
That's a good an explanation as any, because Jackson is bare assertion--it cites to no authority and offers no reasoning for its holding.
The articulation in Boyd is (IMHO) closest to what the framers intended. Boyd approvingly cites the opinion of Lord Camden in Entick v. Carrington and Three Other King's Messengers. In that opinion, Lord Camden explains:
> The justification is submitted to the judges, who are to look into the books, and see if such a justification can be maintained by the text of the statute law, or by the principles of the common law. If no such excuse can be found or produced, the silence of the books is an authority, against the defendant, and the plaintiff must have judgment. According to this reasoning, it is now incumbent upon the defendants to show the law by which this seizure is warranted. If that cannot be done, it is a trespass.
I.e. the government cannot take action that would be a trespass for a private party without a warrant. By that reasoning, your digital information is protected to the extent it is your property. Your Word documents on your hard drive are probably protected. Your tweets probably aren't. Your call-record metadata (which is the cell phone company's property, not yours) is not protected. Nor are your bank records (which are the bank's property, not yours). Pictures of your license plate taken by stoplight cameras are not protected.
[1] Boyd, by the way, sanctions civil forfeiture:
> The seizure of stolen goods is authorized by the common law; and the seizure of goods forfeited for a breach of the revenue laws, or concealed to avoid the duties payable on them, has been authorized by English statutes for at least two centuries past;[†] and the like seizures have been authorized by our own revenue acts from the commencement of the government. The first statute passed by Congress to regulate the collection of duties, the act of July 31, 1789, 1 Stat. 29, 43, contains provisions to this effect. As this act was passed by the same Congress which proposed for adoption the original amendments to the Constitution, it is clear that the members of that body did not regard searches and seizures of this kind as "unreasonable," and they are not embraced within the prohibition of the amendment.