TETRA Radio Code Encryption Has a Flaw: A Backdoor

For more than 25 years, a technology used for critical data and voice radio communications around the world has been shrouded in secrecy to prevent anyone from closely scrutinizing its security properties for vulnerabilities. But now it’s finally getting a public airing thanks to a small group of researchers in the Netherlands who got their hands on its viscera and found serious flaws, including a deliberate backdoor.

The backdoor, known for years by vendors that sold the technology but not necessarily by customers, exists in an encryption algorithm baked into radios sold for commercial use in critical infrastructure. It’s used to transmit encrypted data and commands in pipelines, railways, the electric grid, mass transit, and freight trains. It would allow someone to snoop on communications to learn how a system works, then potentially send commands to the radios that could trigger blackouts, halt gas pipeline flows, or re-route trains.

Researchers found a second vulnerability in a different part of the same radio technology that is used in more specialized systems sold exclusively to police forces, prison personnel, military, intelligence agencies, and emergency services, such as the C2000 communication system used by Dutch police, fire brigades, ambulance services, and Ministry of Defense for mission-critical voice and data communications. The flaw would let someone decrypt encrypted voice and data communications and send fraudulent messages to spread misinformation or redirect personnel and forces during critical times.

Three Dutch security analysts discovered the vulnerabilities—five in total—in a European radio standard called TETRA (Terrestrial Trunked Radio), which is used in radios made by Motorola, Damm, Hytera, and others. The standard has been used in radios since the ’90s, but the flaws remained unknown because encryption algorithms used in TETRA were kept secret until now.

The technology is not widely used in the US, where other radio standards are more commonly deployed. But Caleb Mathis, a consultant with Ampere Industrial Security, conducted open source research for WIRED and uncovered contracts, press releases, and other documentation showing TETRA-based radios are used in at least two dozen critical infrastructures in the US. Because TETRA is embedded in radios supplied through resellers and system integrators like PowerTrunk, it’s difficult to identify who might be using them and for what. But Mathis helped WIRED identify several electric utilities, a state border control agency, an oil refinery, chemical plants, a major mass transit system on the East Coast, three international airports that use them for communications among security and ground crew personnel, and a US Army training base.

Carlo Meijer, Wouter Bokslag, and Jos Wetzels of Midnight Blue in the Netherlands discovered the TETRA vulnerabilities–which they’re calling TETRA:Burst–in 2021 but agreed not to disclose them publicly until radio manufacturers could create patches and mitigations. Not all of the issues can be fixed with a patch, however, and it’s not clear which manufacturers have prepared them for customers. Motorola—one of the largest radio vendors—didn’t respond to repeated inquiries from WIRED.

The Dutch National Cyber Security Centre assumed the responsibility of notifying radio vendors and computer emergency response teams around the world about the problems, and of coordinating a timeframe for when the researchers should publicly disclose the issues.

In a brief email, NCSC spokesperson Miral Scheffer called TETRA “a crucial foundation for mission-critical communication in the Netherlands and around the world” and emphasized the need for such communications to always be reliable and secure, “especially during crisis situations.” She confirmed the vulnerabilities would let an attacker in the vicinity of impacted radios “intercept, manipulate or disturb” communications and said the NCSC had informed various organizations and governments, including Germany, Denmark, Belgium, and England, advising them how to proceed. A spokesperson for DHS’s Cybersecurity and Infrastructure Security Agency said they are aware of the vulnerabilities but wouldn’t comment further.

The researchers say anyone using radio technologies should check with their manufacturer to determine if their devices are using TETRA and what fixes or mitigations are available.

The researchers plan to present their findings next month at the BlackHat security conference in Las Vegas, when they will release detailed technical analysis as well as the secret TETRA encryption algorithms that have been unavailable to the public until now. They hope others with more expertise will dig into the algorithms to see if they can find other issues.

TETRA was developed in the ’90s by the European Telecommunications Standards Institute, or ETSI. The standard includes four encryption algorithms—TEA1, TEA2, TEA3, and TEA4—that can be used by radio manufacturers in different products, depending on their intended use and customer. TEA1 is for commercial uses; for radios used in critical infrastructure in Europe and the rest of the world, though, it is also designed for use by public safety agencies and military, according to an ETSI document, and the researchers found police agencies that use it.

TEA2 is restricted for use in Europe by police, emergency services, military, and intelligence agencies. TEA3 is available for police and emergency services outside Europe—in countries deemed “friendly” to the EU, such as Mexico and India; those not considered friendly—such as Iran—only had the option to use TEA1. TEA4, another commercial algorithm, is hardly used, the researchers say.

The vast majority of police forces around the world, aside from the US, use TETRA-based radio technology, the researchers found, after conducting open source research. TETRA is used by police forces in Belgium and the Scandinavian countries, East European countries like Serbia, Moldova, Bulgaria, and Macedonia, as well as in the Middle East in Iran, Iraq, Lebanon, and Syria.

Additionally, the Ministries of Defense in Bulgaria, Kazakhstan, and Syria use it. The Polish military counterintelligence agency uses it, as does the Finnish defense forces, and Lebanon and Saudi Arabia’s intelligence service, to name just a few.

Critical infrastructure in the US and other countries use TETRA for machine-to-machine communication in SCADA and other industrial control system settings—especially in widely distributed pipelines, railways, and electric grids, where wired and cellular communications may not be available.

Although the standard itself is publicly available for review, the encryption algorithms are only available with a signed NDA to trusted parties, such as radio manufacturers. The vendors have to include protections in their products to make it difficult for anyone to extract the algorithms and analyze them.

To obtain the algorithms, the researchers purchased an off-the-shelf Motorola MTM5400 radio and spent four months locating and extracting the algorithms from the secure enclave in the radio’s firmware. They had to use a number of zero-day exploits to defeat Motorola protections, which they reported to Motorola to fix. Once they reverse-engineered the algorithms, the first vulnerability they found was the backdoor in TEA1.

All four TETRA encryption algorithms use 80-bit keys, which, even more than two decades after their release, still provides sufficient security to prevent someone from cracking them, the researchers say. But TEA1 has a feature that reduces its key to just 32 bits—less than half the key’s length. The researchers were able to crack it in less than a minute using a standard laptop and just four ciphertexts.

Brian Murgatroyd, chair of the technical body at ETSI responsible for the TETRA standard, objects to calling this a backdoor. He says when they developed the standard, they needed an algorithm for commercial use that could meet export requirements to be used outside Europe, and that in 1995 a 32-bit key still provided security, though he acknowledges that with today’s computing power that’s not the case.

Matthew Green, a Johns Hopkins University cryptographer and professor, calls the weakened key a “disaster.”

"I wouldn’t say it’s equivalent to using no encryption, but it’s really, really bad,” he says.

Gregor Leander, a professor of computer science and cryptographer with a security research team known as CASA at Ruhr University Bochum in Germany, says it would be “stupid” for critical infrastructure to use TEA1, especially without adding end-to-end encryption on top of it. “Nobody should rely on this,” he says.

Murgatroyd insists the most anyone can do with the backdoor is decrypt and eavesdrop on data and conversations. TETRA has strong authentication, he says, that would prevent anyone from injecting false communication.

“That’s not true,” says Wetzels. TETRA only requires that devices authenticate themselves to the network, but data and voice communications between radios are not digitally signed or otherwise authenticated. The radios and base stations trust that any device that has the proper encryption key is authenticated, so someone who can crack the key as the researchers did can encrypt their own messages with it and send them to base stations and other radios.

While the TEA1 weakness has been withheld from the public, it’s apparently widely known in the industry and governments. In a 2006 US State Department cable leaked to Wikileaks, the US embassy in Rome describes an Italian radio manufacturer asking about exporting TETRA radio systems to municipal police forces in Iran. The US pushed back on the plan, so the company representative reminded the US that encryption in the TETRA-based radio system they planned to sell to Iran is “less than 40-bits,” implying that the US shouldn’t object to the sale because the system isn’t using a strong key.

The second major vulnerability the researchers found isn’t in one of the secret algorithms, but it affects all of them. The issue lies in the standard itself and how TETRA handles time syncing and keystream generation.

When a TETRA radio contacts a base station, they initiate communication with a time sync. The network broadcasts the time, and the radio establishes that it’s in sync. Then they both generate the same keystream, which is tied to that timestamp, to encrypt the subsequent communication.

“The problem is that the network broadcasts the time in packets that are unauthenticated and unencrypted,” says Wetzels.

As a result, an attacker can use a simple device to intercept and collect encrypted communication passing between a radio and base station, while noting the timestamp that initiated the communication. Then he can use a rogue base station to contact the same radio or a different one in the same network and broadcast the time that matches the time associated with the intercepted communication. The radio is dumb and believes the correct time is whatever a base station says it is. So it will generate the keystream that was used at that time to encrypt the communication the attacker collected. The attacker recovers that keystream and can use it to decrypt the communication collected earlier.

To inject false messages, he would use his base station to tell a radio that the time is tomorrow noon and ask the radio to generate the keystream associated with that future time. Once the attacker has it, he can use the keystream to encrypt his rogue messages, and the next day at noon send them to a target radio using the correct keystream for that time.

Wetzels imagines Mexican drug cartels could use this to intercept police communications to eavesdrop on investigations and operations or deceive police with false messages sent to radios. The attacker needs to be near a target radio, but the proximity is only dependent on the strength of the rogue base station’s signal and the terrain.

“You can do this within a distance of tens of meters,” he says. The rogue base station would cost $5,000 or less to build.

ETSI’s Murgatroyd downplays the attack saying TETRA’s strong authentication requirements would prevent a non-authenticated base station from injecting messages. Wetzel disagrees, saying TETRA only requires devices to authenticate to the network, not to each other.

The researchers didn’t find any weaknesses in the TEA2 algorithm used by police, military, and emergency services in Europe, but they did initially think they found another backdoor in TEA3. Given that TEA3 is the exportable version of TEA2, there was good reason to believe it might also have a backdoor to meet export requirements.

They thought they found something suspicious in a substitution box, or S-box, used in the algorithm, which contains a bad property they say would “never appear in serious cryptography.” The researchers didn’t have sufficient skill to examine it to determine if it was exploitable. But Leander’s team did examine it and he says it’s not.

“In many ciphers if you used such a box it would break the cipher very badly,“ he says. “But the way it’s used in TEA3, we couldn’t see that this is exploitable.” This doesn’t mean someone else might not find something in it he says, but he’d “be very surprised if it leads to an attack that’s practical.”

With regard to fixes for the other problems the researchers found, Murgatroyd says ETSI fixed the keystream/timestamp issue in a revised TETRA standard published last October, and they created three additional algorithms for vendors to use, including one that replaces TEA1. Vendors have created firmware updates that fix the keystream/timestamp issue. But the problem with TEA1 cannot be fixed with an update. The only solution for that is to use another algorithm—not an easy thing to switch—or to add end-to-end encryption on top of TETRA, something Wetzels says is impractical. It’s very expensive since the encryption has to be applied to every device, it requires some downtime to do the upgrade—something not always feasible for critical infrastructure—and can create incompatibility issues with other components.

As for asking their vendor to switch out TEA1 for one of the new algorithms meant to replace it, Wetzels says this is problematic as well, since ETSI plans to keep those algorithms secret, like the others, asking users to trust again that the algorithms have no critical weakness.

“There’s a very high chance that [the replacement algorithm for TEA1] will be weakened” as well, he says.

The researchers don’t know if the vulnerabilities they found are being actively exploited. But they did find evidence in the Edward Snowden leaks that indicate the US National Security Agency (NSA) and UK’s GCHQ intelligence agency targeted TETRA for eavesdropping in the past. One document discusses an NSA and Australian Signals Directorate project to collect Malaysian police communications during a climate change conference in Bali in 2007 and mentions that they obtained some TETRA collections on Indonesian security forces’ communications.

Another Snowden leak describes GCHQ, possibly with NSA assistance, collecting TETRA communications in Argentina in 2010 when tensions rose between it and the UK over oil exploration rights in a deep-sea oil field off the coast of the Falkland Islands. It describes an operation to collect high-priority military and leadership communications of Argentina and reveals that the project resulted in successful TETRA collections.

“This doesn’t indicate they exploited these vulnerabilities that we found,” Wetzels says. “But it does show … that state-sponsored actors are actively looking at and collecting these TETRA networks, even in the early 2000s.”