The Supreme Court has announced, in an undivided decision, that police do not have the right to search the data stored on a cell phone without a warrant, just because the phone was seized as a result of an arrest. A cellphone is not comparable to an arrestee's wallet, as many government officials argued. (Riley v California)

This opinion is extremely important because almost everyone carries a cellphone with private photos, messages, and other data. The Court even stated that cell phones in today's day and age are such a prevalent part of our daily lives “that the proverbial visitor from Mars might conclude they were an important feature of human anatomy.” The Court's decisions will also influence Fourth Amendment regulations in vital privacy and surveillance cases from now on.

The government emphasized the idea that searching an arrestee's cellphone is just as if an officer is searching inside an arrestee's pockets, or examining a cigarette pack found in his possession, as approved in United States v. Robinson 414 US 218 (1973) .

So, in the case of the NSA and the bulk collection of call detail records and other metadata, what does this mean? The court wants to save that question for another day. The court stated,

Because the United States and California agree that these cases involve searches incident to arrest, these cases do not implicate the question whether the collection or inspection of aggregated digital information amounts to a search under other circumstances.

Based on what the Court has said, the government's approach may not win the day. So far, the government and the FISC (Foreign Intelligence Service Court) have said that the 1979 case of Smith v. Maryland—approving warrantless installation of a “pen register” to capture numbers dialed—allows warrantless collection of metadata in general, and call detail records in particular. The government also relied on Smith in Riley, arguing that a cell phone call log is just like the numbers which a pen register captures. The Court rejected this argument.

The Court stated that the information on a cell phone is very different from the items which people generally have in their possession:

Cell phones differ in both a quantitative and a qualitative sense from other objects that might be kept on an arrestee's person. The term “cell phone” is itself misleading shorthand; many of these devices are in fact minicomputers that also happen to have the capacity to be used as a telephone. They could just as easily be called cameras, video players, rolodexes, calendars, tape recorders, libraries, diaries, albums, televisions, maps, or newspapers.

The government tried to save warrantless local searches of devices by suggesting that law enforcement agencies “develop protocols to address” concerns raised by cloud computing. The Court rejected this idea.

The Court also raised the point that the Fourth Amendment requires a warrant for long range GPS installation and tracking. The Supreme Court held that searching a person's phone is considered a violation of privacy because of all of the data stored in it. Fortunately, the Riley decision has maintained privacy protections under the Fourth Amendment.