Code That Protects Most Cellphone Calls Is Divulged

BERLIN — A German computer engineer said Monday that he had deciphered and published the secret code used to encrypt most of the world’s digital mobile phone calls, in what he called an attempt to expose weaknesses in the security of global wireless systems.

The action by the encryption expert, Karsten Nohl, aimed to question the effectiveness of the 21-year-old Global System for Mobile Communications (GMS –  Groupe Spécial Mobile), a code developed in 1988 and still used to protect the privacy of 80 percent of mobile calls worldwide.

“This shows that existing GSM security is inadequate,” Mr. Nohl, 28, told about 600 people attending the Chaos Communication Congress, a four-day computer hackers’ conference that runs through Wednesday in Berlin. “We are trying to push operators to adopt better security measures for mobile phone calls.”

The GSM Association, the industry group based in London that devised the algorithm and represents wireless operators, called Mr. Nohl’s efforts illegal and said they overstated the security threat to wireless calls.

“This is theoretically possible but practically unlikely,” said Claire Cranton, a GSM spokeswoman, noting that no one else had broken the code since its adoption. “What he is doing would be illegal in Britain and the United States. To do this while supposedly being concerned about privacy is beyond me.”

Some security experts disagreed. While the disclosure does not by itself threaten the security of voice data, one analyst said companies and governmental organizations should take the same steps to ensure the security of their wireless conversations as they do with antivirus software for computer files.

“Organizations must now take this threat seriously and assume that within six months their organizations will be at risk unless they have adequate measures in place to secure their mobile phone calls,” said Stan Schatt, a vice president for health care and security at the technology market researcher ABI Research in New York.

Mr. Nohl, who has a doctorate in computer engineering from the University of Virginia, is a widely consulted encryption expert who waged a similar campaign this year that prodded the DECT Forum, a standards group based in Bern, to upgrade the security algorithm for 800 million cordless home phones.

Mr. Nohl has now set his sights on GSM, whose second-generation digital technology is still the most widely used wireless-communications standard in the world. About 3.5 billion of the world’s 4.3 billion wireless connections use GSM; it is used by about 299 million consumers in North America.

In August, at a hackers’ forum in Amsterdam, Mr. Nohl challenged other computer hackers to help him crack the GSM code. He said about 24 people, some members of the Chaos Computer Club, which is based in Berlin, worked independently to generate the necessary volume of random combinations until they reproduced the GSM algorithm’s code book — a vast log of binary codes that could theoretically be used to decipher GSM phone calls.

The code book, Mr. Nohl said, contains the equivalent of about two terabytes, or 2,000 gigabytes, of digital information, the equivalent of 100 high-definition films.

During an interview, Mr. Nohl said he took precautions to remain within legal boundaries, emphasizing that his efforts to crack the GSM algorithm were purely academic, kept within the public domain, and that the information was not used to decipher a digital call.

“We are not recommending people use this information to break the law,” Mr. Nohl said. “What we are doing is trying to goad the world’s wireless operators to use better security.”

Mr. Nohl said the algorithm’s code book was available on the Internet through services like BitTorrent, which some people use to download vast quantities of data like films and music. He declined to provide a Web link to the code book, for fear of the legal implications, but said its location had spread by word of mouth through the hackers’ community.

The GSM algorithm, technically known as the A5/1 privacy algorithm, is a binary code — which is made exclusively of 0’s and 1’s — that has kept digital phone conversations private since the GSM standard was adopted in 1988.

But the A5/1 algorithm is a 64-bit binary code, the modern standard at the time it was developed, but simpler than the 128-bit codes used today to encrypt calls on third-generation networks. The new codes have twice as many 0’s and 1’s.

In 2007, the GSM developed a 128-bit successor to the A5/1, called the A5/3 (KASUMI) encryption algorithm, but most network operators have not yet invested to make the security upgrade.

The disclosure of a GSM encryption key, in and of itself, does not enable surveillance of mobile calls, which must still be overheard and identified from the digital stream of thousands of calls transmitted through a single cell phone station.

The undertaking is highly complex because a digital call typically hops among up to 60 different broadcast frequencies during a single conversation, as the mobile network operator maximizes the use of its available bandwidth.

In a statement, the GSM Association said efforts to crack the algorithm were more complex than critics have asserted, and that operators, by simply modifying the existing algorithm, could thwart any unintended surveillance.

“We strongly suspect that the teams attempting to develop an intercept capability have underestimated its practical complexity,” GSM said in a statement. The association noted that hackers intent on illegal eavesdropping would need a radio receiver system and signal processing software to process raw radio data, much of which is copyrighted.

But Mr. Nohl, during a presentation Sunday to attendees at the Berlin conference, said the hardware and software needed for digital surveillance were available free as an open-source product in which the coding is available for individuals to tailor to their needs.

A security expert whose company sells software to governments, businesses and aid agencies that provides extra layers of security for wireless calls, said Mr. Nohl’s disclosures highlighted the need for better wireless security.

Simon Bransfield-Garth, the chief executive of Cellcrypt, based in London, said Mr. Nohl’s efforts could put sophisticated mobile interception technology — limited to governments and intelligence agencies — within the reach of “any reasonable well-funded criminal organization.”

“This will reduce the time to break a GSM call from weeks to hours,” Mr. Bransfield-Garth said during an interview. “We expect as this further develops it will be reduced to minutes.”

Mr. Bransfield-Garth said advances in surveillance technology have made it possible to buy commercial wireless surveillance systems in countries like India for as little as $1,500.

“Customers have told us they have lost multimillion-dollar deals because of information that has leaked out to competitors through phone intercepts,” Mr. Bransfield-Garth said. “Phone intercepts are considerably more common than people realize.”


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