T-Mobile quietly hardens part of its US cellular network against snooping

Wireless carrier T-Mobile US has been quietly upgrading its network in a way that makes it harder for surveillance equipment to eavesdrop on calls and monitor texts, even on the company’s legacy system.

The upgrade involves switching to a new encryption standard, called A5/3, that is harder to crack than older forms of encryption. Testing by The Washington Post has found T-Mobile networks using A5/3 in New York, Washington and Boulder, Colorado, instead of the older A5/1 that long has been standard for second-generation (2G) GSM networks in the United States. More advanced technologies, such as 3G and 4G, already use stronger encryption. In places where T-Mobile is using A5/3 encryption, mass surveillance becomes more difficult because equipment that passively collects cellular signals from the air often cannot decode calls. Active attacks, involving a device called an “IMSI catcher,” may still be able to eavesdrop on individual calls by manipulating a phone’s security settings directly, without having to crack the encryption.