Photographer: John Moore/Getty Images
 Photographer: John Moore/Getty Images

During this week's Supreme Court arguments about the legality of mobile-phone searches by police officers, the justices were searching for a compromise. Because criminals use mobile phones, police want to know whether an arrestee has just texted for backup. Warrant requirements delaying that would feel burdensome.

At the same time, because a complete dossier of our lives is accessible by way of a phone, failing to have any judicial oversight of wide-ranging police access to them -- even at the time of arrest -- would be wrong.

Unfortunately, some limiting principle that will allow law enforcement to dig through a phone's contents while not exposing individuals to unfettered searches without legal oversight may not be available. That's because the cases this week concerned a primitive version of a deep question: When most human activities involve screens and sensors that automatically save data on remote computers, what legal standards should constrain law enforcement's access to that distant information?

It's essential that we get this right, because mobile phones are just plastic boxes that gather and generate data for distant use. What's important about the handset is the link it provides between a person and all that data.

Beyond accessing immediate, on-the-spot communications to protect the safety of arresting officers, any further examination of a phone at the time of an arrest should be made by law enforcement only under the supervision of a judicial officer.

From the user's perspective, data stored on a phone is the same as data stored on remote cloud servers and available by way of the handset. According to Deloitte LLP, by the end of 2013 there were an estimated 600 million personal cloud subscriptions, a number that will double by the end of 2017. Law enforcement authorities are interested in phones because they have master keys: access to the identities and passwords that were used to set up and connect all these subscriptions.

That can lead to opening troves of data about health conditions, financial records, personal relationships and every other form of human interaction. So it's clear that thinking of a mobile phone as the digital-age descendant of a briefcase doesn't make sense.

When Deputy Solicitor General Michael R. Dreeben suggested that mobile phones are carrying the same kind of information that previously would have been available in paper form, Justice Anthony Kennedy didn't buy the parallel: "I don't think it's odd to say that we're living in a new world," he said. He's right. Phones are keys to more data, and more profoundly personal background information, than ever could have been accessed by the most diligent shoe-leather investigation.

Still, the government's lawyers have a strong argument: They say they need warrantless searches of handsets at the time of arrest because they're worried about missing quick access to information that, if known, could keep them safe. There may be instances in which phones can be used by arrestees to trigger dangers or call in accomplices. Access to a phone in order to check immediate outgoing communications in the context of an arrest, even without a warrant, seems entirely appropriate.

But any broader rummaging through phones would amount to the kind of unconstrained search that the framers had in mind while drafting the Fourth Amendment. If the police are worried that phones will be remotely "wiped," let's suggest that they wrap aluminum foil around the phones they sweep up, or take out the batteries. Otherwise, they are getting far more access to intimate details of arrestees' lives much more easily than the framers could possibly have imagined.

Dreeben tried to downplay the importance of the mobile-phone-search question, saying that "it may well be that in the future more information will migrate to the cloud, less will be on the phone, and that may shift what the officers can actually do." That's misleading: The point is that access to a handset makes it possible to unlock mountains of cloud-based information.

Justice Elena Kagan noted the problem. "I thought the whole ideas of smartphones, Mr. Dreeben, and increasingly so, was that even the user doesn't know what what's on the cloud or not," she said. An officer with handset-saved keys to the cloud kingdom will know in a moment almost anything about a user's life.

Law enforcement officials often maintain that the advent of the digital era means they need broad new authority to track and apprehend criminals. They call the digitization of information "going dark." But the reality of mobile-phone technology, and the power we humans allow mobile phones to have over our life patterns, belies this claim. Rather than going dark, these digital times are allowing law enforcement to shed more light on human lives than ever before. We shouldn't let this happen without independent judicial oversight.

To contact the writer of this article: Susan P. Crawford at scrawford@scrawford.net.

To contact the editor responsible for this article: Christopher Flavelle at cflavelle@bloomberg.net.