WASHINGTON — The cellphone encryption technology used most widely around 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 are 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.

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.