Karsten Nohl: Finding the Flaws in Everyday Technology

Karsten Nohl has a habit of picking the one piece of technology everybody assumes is fine and taking it apart in public.

The transit card in your wallet, the encryption on your phone calls, the SIM card inside the phone, the USB stick on your keychain, the signaling network that connects every carrier on earth: one by one, the Berlin cryptographer has shown that each was built on secrets that couldn't survive being looked at.

Then, in 2016, a television crew handed him a congressman's phone number.

Karsten Nohl at a Glance

• A German engineer from the Rhineland, Nohl studied electrical engineering in Heidelberg and earned a PhD in computer engineering at the University of Virginia in 2008.
• In December 2007 he and Henryk Plötz showed the Chaos Communication Congress how they'd found the weaknesses in the secret cipher inside Mifare Classic, the world's most widely used smart card chip.
• Two years later he was back with "GSM: SRSLY?", a talk announcing that the encryption protecting three billion mobile phones could be cracked with downloadable tables.
• In 2010 he founded Security Research Labs in Berlin, a consultancy whose paying work funds research it publishes for free.
• In 2013 he revealed that hundreds of millions of SIM cards could be taken over with a text message, and in 2014 he and Jakob Lell unveiled BadUSB, proving that any USB device could be reprogrammed into a weapon.
• That December his team demonstrated attacks on SS7, the global phone-routing network, and in 2016 he tracked and intercepted Congressman Ted Lieu's phone from Berlin for 60 Minutes.
• SRLabs now employs more than fifty researchers in Berlin and Hong Kong and belongs to the European security group Allurity, where Nohl is chief innovation officer.

The Life of Karsten Nohl

Nohl's Early Years

Karsten Nohl grew up in Germany's Rhineland and studied electrical engineering in Heidelberg before crossing the Atlantic for graduate school. At the University of Virginia he earned a PhD in computer engineering in 2008, working on the security of RFID chips, the tiny radio tags in transit cards and building passes.

What fascinated him, he later said, was the place where hardware and software meet, and the cryptography that lives there.

Mifare

In December 2007, still a graduate student, Nohl walked onto the stage of the Chaos Communication Congress in Berlin with Henryk Plötz. Their talk was titled simply "Mifare."

Mifare Classic chips were, as they put it, "the most widely deployed brand of secure RFID chips, but their security relies on proprietary and secret cryptographic primitives." The two had analyzed the chip's hardware and found weaknesses throughout the cipher.

The lesson would become Nohl's signature: secrecy isn't security. A cipher nobody is allowed to examine is usually a cipher nobody has examined.

GSM: SRSLY?

Two years later, in December 2009, he was back at the Congress with the American researcher Chris Paget and a talk called "GSM: SRSLY?" The world's most popular radio system, their abstract noted, "has over 3 billion handsets in 212 countries and not even strong encryption."

The target was A5/1, the cipher that scrambled calls on most of the planet's mobile networks. Nohl's project had computed huge lookup tables that turned cracking it from a job for intelligence agencies into something a hobbyist could do with downloadable data and ordinary hardware.

Carriers grumbled. The tables spread anyway.

Security Research Labs

In 2010 Nohl founded Security Research Labs, or SRLabs, in Berlin. Its business model is unusual: "We test systems and we publish what we find. The consulting pays for the research."

The research kept coming. In July 2012 Nohl and his colleague Thomas Roth showed they could take over a VeriFone payment terminal used by some 300,000 shops in Germany, and made the point by getting one to play Pong.

"Without some drastic publicity," Nohl said, "I don't think that shopkeepers will know about it."

Rooting the SIM

The SIM card had always been treated as the one trustworthy thing in a phone. Over two years Nohl's team tested more than 1,000 of them and found that many still relied on DES, a cipher from the 1970s, to protect the over-the-air messages carriers use to update them.

A specially crafted text message could trick a card into giving up its key. "The hacker can redirect calls, rewrite numbers, listen in on calls," Nohl told the Guardian.

Forbes broke the news ten days early under the headline "SIM Cards Have Finally Been Hacked." He presented the full research at Black Hat in Las Vegas on July 31, 2013, at the age of 31, "startling peers who had considered sim cards to be relatively safe technology." Hundreds of millions of phones were affected.

BadUSB

In the summer of 2014 Nohl and Jakob Lell went after something humbler. USB devices, from thumb drives to keyboards, run on firmware that almost nobody checks and that can be rewritten.

Their proof of concept, BadUSB, turned a thumb drive into a device that could pose as a keyboard and type commands, redirect a computer's internet traffic or infect other drives plugged into the same machine. "These problems can't be patched," Nohl told Wired. "We're exploiting the very way that USB is designed."

An infected drive could infect a PC, and an infected PC could infect every drive it touched. "It goes both ways," Nohl said. "Nobody can trust anybody."

The Black Hat talk has been watched well over 100,000 times on the conference's own channel, and within two months other researchers had published working code.

The Network Itself

That December, at the Chaos Communication Congress in Hamburg, Nohl's team turned to SS7, the decades-old signaling system carriers use to route calls and texts between networks, and showed how its trust-everyone design let an outsider track and intercept a phone from anywhere. The demonstration target was a German senator's phone.

Most people heard about it on April 17, 2016, when 60 Minutes aired a segment in which Nohl, working from Berlin with nothing but a phone number, located and intercepted the calls of California congressman Ted Lieu, who'd agreed to be the guinea pig.

"All phones are the same," Nohl told correspondent Sharyn Alfonsi. The network, he explained, "knows where you are," whatever you do with the GPS chip. Lieu's verdict on a known flaw left open for intelligence agencies: "That is not acceptable."

Nohl later explained the location trick to IEEE Spectrum in one line: "Your phone only says 'Hi' once every six hours where we can say 'Hi' every minute so we can dominate that ping pong game."

Around the same time Nohl took a side job few hackers get, an interim stint as chief information security officer of a large Asian mobile operator, seeing the problems from the other side of the table.

Nohl Today

SRLabs has grown to more than fifty researchers, hackers and engineers in Berlin and Hong Kong, and the mobile networks it's tested serve more than 400 million subscribers. The firm is now part of Allurity, a European cybersecurity group with offices in 18 countries, and Nohl, having run SRLabs as chief executive and chief scientist, is the group's chief innovation officer.

He's still the person reporters call when a cipher wobbles. When researchers announced in September 2026 that they'd bypassed 1,024-bit RSA signatures without factoring the key, it was Nohl who told Gizmodo: "If this result holds up under peer review, it would indeed be a conceptual break-through."

His recent writing is about AI, and the argument is familiar: when attackers upgrade their tools every month, an annual penetration test can't keep up.

Karsten Nohl: Nobody Can Trust Anybody

Nohl's career is a single experiment repeated on bigger and bigger systems: find the component everyone trusts because nobody has looked, and look. The transit card, the phone cipher, the SIM, the USB stick and the global signaling network all failed the same test.

The uncomfortable part is how little changed afterward. SS7 is still in use; USB firmware is still unsigned. His research tends to become a permanent asterisk rather than a fix.

Which is why, two decades on, his phone still rings when something supposedly unbreakable breaks.