WASHINGTON - The cellphone encryption technology used most widely across the world can be easily defeated by the National Security Agency, an internal document shows, giving the agency the means to decode most of the billions of calls and texts that travel over public airwaves every day.
While the military and law enforcement agencies long have been able to hack into individual cellphones, the NSA's capability appears to be far more sweeping because of the agency's global signals collection operation. The agency's ability to crack encryption used by the majority of cellphones in the world offers it wide-ranging powers to listen in on private conversations.
U.S. law prohibits the NSA from collecting the content of conversations between Americans without a court order. But experts say that if the NSA has developed the capacity to easily decode encrypted cellphone conversations, then other nations likely can do the same through their own intelligence services, potentially to Americans' calls, as well.
Encryption experts have complained for years that the most commonly used technology, known as A5/1, is vulnerable and have urged providers to upgrade to newer systems that are much harder to crack. Most companies worldwide have not done so, even as controversy has intensified in recent months over NSA collection of cellphone traffic, including of such world leaders as German Chancellor Angela Merkel.
The extent of the NSA's collection of cellphone signals and its use of tools to decode encryption is not clear from a top-secret document provided by former contractor Edward Snowden. But it states that the agency "can process encrypted A5/1" even when the agency has not acquired an encryption key, which unscrambles communications so that they are readable.
Experts say the agency may also be able to decode newer forms of encryption, but only with a much heavier investment in time and computing power, making mass surveillance of cellphone conversations less practical.
"At that point, you can still listen to any [individual person's] phone call, but not everybody's," said Karsten Nohl, chief scientist at Security Research Labs in Berlin.
The vulnerability outlined in the NSA document concerns encryption developed in the 1980s but still used widely by cellphones that rely on technology called second-generation (2G) GSM. It is dominant in most of the world but less so in the wealthiest nations, including the United States, where newer networks such as 3G and 4G increasingly provide faster speeds and better encryption, industry officials say.
But even where such updated networks are available, they are not always used, because many phones often still rely on 2G networks to make or receive calls. More than 80 percent of cellphones worldwide use weak or no encryption for at least some of their calls, Nohl said. Hackers also can trick phones into using these less-secure networks, even when better ones are available. When a phone indicates a 3G or 4G network, a voice call might actually be carried over an older frequency and susceptible to decoding by the NSA.
The document does not make clear if the encryption in another major cellphone technology - called CDMA and used by Verizon, Sprint, and a small number of foreign companies - has been broken by the NSA as well. The document also does not specify whether the NSA can decode data flows from cellular devices, which typically are encrypted using different technology.
The NSA has stressed that its data-collection efforts are aimed at overseas targets, whose legal protections are much lower than U.S. citizens'. When questioned for this story, the agency issued a statement, saying: "Throughout history nations have used encryption to protect their secrets, and today terrorists, cyber criminals, human traffickers and others also use technology to hide their activities. The Intelligence Community tries to counter that in order to understand the intent of foreign adversaries and prevent them from bringing harm to Americans and allies."
German news magazine Der Spiegel reported in October that a listening station atop the U.S. Embassy in Berlin allowed the NSA to spy on Merkel's cellphone calls. It also reported that the NSA's Special Collection Service runs similar operations from 80 U.S. embassies and other government facilities worldwide. These revelations - and especially reports about eavesdropping on the calls of friendly foreign leaders - have caused serious diplomatic fallouts for the Obama administration.
Cellphone conversations long have been much easier to intercept than ones conducted on traditional telephones because the signals are broadcast through the air, making for easy collection. Police scanners and even some older televisions once were able to routinely pick up people talking on their cellphone.
Digital transmission and encryption have become almost universally available in the United States, and they are now standard throughout much of the world.
Governments typically dictate what kind of encryption technology, if any, can be deployed by cellphone service providers. As a result, cellular communications in some nations, including China, feature weak or no encryption.
A5/1 has been repeatedly cracked by researchers in demonstration projects for more than a decade.
Matthew Blaze, a University of Pennsylvania cryptology expert, said the weakness of A5/1 encryption is "a pretty sweeping, large vulnerability" that helps the NSA listen to cellphone calls overseas and likely also allows foreign governments to listen to the calls of Americans.