Research May Hasten Death of Mobile Privacy Standard

Researchers at a computer security conference in Washington, D.C. this week detailed a method for dramatically reducing the cost and time needed to crack the security that prevents eavesdropping of GSM-based mobile phones.

The weaknesses in the GSM encryption technology — a 64-bit scheme known as A5/1 — were first detailed nearly a decade ago, but cracking the code has generally required a great deal of patience and some very expensive hardware (with hardware costs alone exceeding $1 million). U.S. based GSM carriers — including AT&T and T-Mobile — as well as most European GSM providers are among the dozens of mobile providers and billions of handsets worldwide using A5/1 as their privacy standard.

Most of the previously detailed methods for cracking A5/1 encrypted GSM communications involved “active attacks,” injecting data packets into the carrier’s system or circumventing the encryption altogether by tricking a nearby target’s phone into connecting to a bogus, unencrypted relay station controlled by the attacker. But researchers David Hulton and Steve Miller say their method relies on a purely passive attack, which can be done remotely and takes advantage of massive advances in parallel computing power to crunch through a listing of all possible GSM encryption keys in a matter of minutes.

The duo’s new discovery means the ability to hack into one of these devices could be easier (and more affordable) for both government agencies, law enforcement, hobbyists and would-be thieves.

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GSM Interception

GSM Interception

21.11.1999

Lauri Pesonen
Department of Computer Science and Engineering
Helsinki University of Technology
Lauri.Pesonen@iki.fi

Abstract

The GSM standard was designed to be a secure mobile phone system with strong subscriber authentication and over-the-air transmission encryption. The security model and algorithms were developed in secrecy and were never published. Eventually some of the algorithms and specifications have leaked out. The algorithms have been studied since and critical errors have been found. Thus, after a closer look at the GSM standard, one can see that the security model is not all that good. An attacker can go through the security model or even around it, and attack other parts of a GSM network, instead of the actual phone call. Although the GSM standard was supposed to prevent phone cloning and over-the-air eavesdropping, both of these are possible with little additional work compared to the analog mobile phone systems and can be implemented through various attacks. One should not send anything confidential over a GSM network without additional encryption if the data is supposed to stay confidential.

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