MOBILE operators like to trumpet the speed of their fastest networks. Third-generation (3G) service is now old hat, and 4G networks—whether legitimately labeled as such or not—are the latest fad. Meanwhile, slow-and-steady 2G GSM-based service quietly remains the most widely used cell technology worldwide. That is a problem, according to Karsten Nohl, a member of a research team that has cracked the encryption protocol used for most of the data sent and received around the globe. (The researchers have a technical presentation available for download.)

This decryption effort enables outsiders to eavesdrop on data connections or voice calls placed over a 2G network in a jiffy. Dr Nohl says that his team's test laptop, a reasonably powerful modern machine, may crack a call in 11 minutes using just €10 ($14) in radio components. The crack must be repeated for every connection, however; it does not allow unfettered and continuous access to all conversations and information sent on 2G networks. But it does make it possible for specific sessions to be intercepted, making the method useful in targeting particular activities, businesses or individuals.

Dr Nohl stresses that the 11 minutes was just a first pass at writing the cracking software, and that his group used only modest equipment with no financial motive. Criminals, by contrast, could benefit mightily from accelerating the crack, he says, one reason his group has refrained from expounding the technique in detail. It has, however, pointed to some specific holes which ought to be plugged. The group found some networks disabled all security features, relying on the highly misguided notion that traffic could not be easily intercepted except by mobile operators. Having no security from the phone to a base station on a mast makes it easier to filter and monitor traffic.

In 2009 Dr Nohl and colleagues pointed out significant weaknesses to the base GSM standard. Their new attack focuses on General Packet Radio Service, better known as GPRS—a modest improvement to GSM—introduced commercially in 2000. GPRS allows rates of tens of kilobits per second (Kbps), while a subsequent tweak known as EDGE allows downstream rates of 200 to 400 Kbps. GPRS and EDGE are commonly referred to as 2.5G, sitting in between 2G and 3G network speeds.

Over 5 billion GSM mobile subscriptions are active worldwide, but just over 700m of those have access to 3G service, according to Informa Telecoms and Media, a research firm. And 3G service is an add-on to existing 2G networks, not a fully fledged replacement. A 3G phone can conserve energy and free up 3G spectrum for internet use by switching to 2G for voice calls and slow data connections. GPRS and EDGE remain dominant outside metropolitan areas and in developing countries. Several hundred million subscriptions are for all-important machine-to-machine connections, such as ATMs authenticating bank transactions, smart-meter reporting, vending machines sending automated orders for more cans and bottles, or remote alarm monitoring. Nokia Siemens Networks, a maker of communications kit, estimates that this figure will rise to one billion by 2015. "It really is the information backbone of what people call the smart society," says Dr Nohl.

The industry's trade group has taken previous disclosures seriously. Operators can take measures to upgrade security on some phones. But network-wide upgrades will do little to keep basic phones and smartphones secure. Fortunately, GPRS's security flaws can be fixed by using encryption enabled in software that runs on individual phones, either through apps or the device's operating system. Apps could, for instance, enable encrypted web browser sessions, which have become available on desktop browsers as an option for all Facebook and Twitter traffic, as well as being an absolute requirement for some time on e-commerce, banking and investment sites. In fact, many smartphone apps may already silently employ encryption to protect data over both mobile and Wi-Fi networks. Surprisingly, app makers have so far been loth to trumpet such measures, even though the cost of such security is low in most cases.

Additional encryption would not prevent perpetrators from penetrating GPRS connections. But such intrusions would net only scrambled bits. ATM and other machine-to-machine networks already tend to use encryption anyway, but Dr Nohl notes that some companies that run them may put too much faith in the mobile network's security. This is becoming increasingly unwise.

The silver lining is that 3G networks boast far superior security protocols. Dr Nohl's team has examined these faster and more sophisticated networks, too. So far, they have found no glaring flaws. 3G networks rely on heavily scrutinised algorithms which are publicly available so anyone who spots a potential problem can signal it immediately. That said, 3G will not replace 2G networks in most countries for well over a decade. Before that happens, additional cryptographic protection is the only safe bet.