A flaw in encryption technology used in some SIM cards could leave millions of phones exposed to spying according to Karsten Nohl.

The vulnerability allows attackers to send a spoof text message to the device which in return reveals the 56-bit data encryption standard key (DES). With this key, the attacker could install malicious software on the device and have the ability to listen in on your phone calls, access/send text messages and much more in just 2 minutes.

About half of the SIM cards today still rely on the older DES encryption rather than a more secure triple-DES encryption. However, Nohl was able to access around 25% of SIM cards in his testing. He estimates that 750 million phones could be affected by this vulnerability.

Give me any phone number and there is some chance I will, a few minutes later, be able to remotely control this SIM card and even make a copy of it

Nohl described the attack in much more detail as well.

In early 2011, Nohl’s team started toying with the OTA protocol and noticed that when they used it to send commands to several SIM cards, some would refuse the command due to an incorrect cryptographic signature, while a few of those would also put a cryptographic signature on this error message.

With that signature and using a well known cryptographic method called rainbow tables, Nohl was able to crack the encryption key on the SIM card in about one minute. Carriers use this key to remotely program a SIM, and it is unique to each card.

“Anybody who learns the key of a particular SIM can load any application on the SIM he wants, including malicious code,” says Jasper Van Woudenberg, CTO North America of smart-card security firm Riscure.

“We had almost given up on the idea of breaking the most widely deployed use of standard cryptography,” says Nohl, but it felt “great” to finally gain control of a SIM after many months of unsuccessful testing.

With the all-important (and till-now elusive) encryption key, Nohl could download a virus onto the SIM card that could send premium text messages, collect location data, make premium calls or re-route calls. A malicious hacker could eavesdrop on calls, albeit with the SIM owner probably noticing some suspiciously-slow connections.

Nohl was also the security researcher who exposed GSM's weak encryption that allowed anyone with the right tools to listen in on cellphone calls. As a result of his work the systems used to encrypt GSM calls were strengthened. Nohl believes carriers must phase out SIMs using DES and implement better filter technology to block spoofed messages.