TETRA:BURST

TETRA:BURST

“Don’t Cook Your Own Crypto” and “Don’t Cripple Its Security”

Anyone can create a cipher. Basically, Bob and Alice do some modulo maths and could encrypt their secret messages into ciphertext by multiplying by 10 and adding 5, and then to decrypt back into plaintext, they would just subtract the ciphertext by 5 and divide by 10. The maths involved could then be defined by a Galois Field (GF)— and which is named after Évariste Galois. Bob and Alice could then keep their method secret from Eve (their adversary), and where they believe their method is secure and thus do not ask Trent to evaluate its security.

But Eve is sneaky and tries lots of different ways to crack the cipher. Eventually, after trying to crack the ciphertext, she discovers the method, and can then crack all the future (and, possibly, previous) ciphers. Bob and Alice then carry on using the secret cipher method and would then have no way of knowing that Eve now knows their method.

This approach is often known as “cooking your own crypto”, and is not recommended in most implementations. Along with this, as Bob and Alice try to hide their method from Eve, the approach is “Security by obfuscation” rather than “Security-by-design”.

Cooking your own crypto

There are many cases of propriety cryptography methods being used in production. In 2013, for example, researchers at the University of Birmingham found flaws in the key fobs related to the Volkswagen group vehicles. In fact, the encryption used in the Swiss-made Megamos transponder was so weak that an intruder only needed to listen to two transmitted messages from the fob in order to crack the key.

The vulnerability related to the poor, proprietary cryptographic methods used by the device, and where the researchers found they could generate the transponder’s 96-bit secret key and start the car in less than half an hour. The vulnerability has been well known since 2012, and code to exploit the flaw has circulated online since 2009. Yet, at the time, there was no product recall for the dozens of models that were affected, including Audi, Porsche, Bentley and Lamborghini, Nissan and Volvo. The research team were even stopped from publishing their work through the threat of legal action from Volkswagen.

Testing, Evaluation and Standardization

Along with the risk of discovering a secret method, the other major problem is that the method used to create a cipher is when it is not rigorously reviewed by experts. This can take years of reviewing and testing — both in the formal theory and in practice. Many companies, too, have bug bounties and which try to discover vulnerabilities in their code. To overcome this, NIST has created open competitions for the standardization of encryption methods. These have included standards related to symmetric key encryption (AES), hashing methods (SHA-3) and post-quantum cryptography (PQC). Once rigorously evaluated, the industry can then follow the standards defined, and where proprietary methods and implementations are often not trusted.

With symmetric-key methods (where the same key is used to both encrypt and decrypt), at one time, we used a wide range of methods, such as DES, 3DES, RC2, RC4, Blowfish, and Twofish. To overcome this, NIST set up an operation standardization process for the Advanced Encryption Standard (AES). In the end, and after extensive testing and performance analysis, the Rijndael method was selected. It is now used in most systems, with either a 128-bit, a 192-bit or 256-bit encryption key. Overall, the larger the key size, the more difficult it is to brute force the key.

The TETRA standard

This week it has been reported that the TETRA (TErrestrial Trunked RAdio) standard [here] has a number of vulnerabilities in its cryptography. Overall, TETRA is used by many police and military forces across the world for encrypted radio. These vulnerabilities have existed for over a decade and could have led to the leakage of sensitive information.

These vulnerabilities have been discovered by Midnight Blue and will be presented as “Redacted Telecom Talk” at Black Hat 2023 on 9 August 2023 [here]. As the work is so sensitive, there are many issues related to its disclosure, so the full details of the talk have not been released. But, it has involved over 18 months of responsible disclosure related to the cracking of TETRA-powered radios purchased from eBay.

TETRA was first standardised by the European Telecommunications Standards Institute (ETSI) in 1996 and used by many radio manufacturers, such as Motorola and Airbus. It does not have open-source software and relies on cryptography which is secret and proprietary.

TEA1 — Intentionally weak crypto

Goverments around the world have generally used export controls on cryptography — in order to reduce security levels so that their own law enforcement agents have a good chance to crack encrypted traffic outside their own borders. One of the most famous was related to Netscape and who created the original version of TLS (Transport Layer Security) that created a secure channel for Web pages — the HTTPs that we see on most of our Web accesses now.

This, though, had reduced security levels because of export control — with the RSA method used set at only 512 bits (and which is now easily crackable). As this key was used to pass the encryption key that was used in the secure tunnel, it meant that agencies could break the communications channel for HTTPs communications. We have since paid for this weakening —and with vulnerabilities such as Freak and BEAST. The vulnerability in TETRA, too, relates to similar issues and where the cryptography was reduced to comply with export controls. Within TERTA, the TEA1 method reduces the key size down to 80 bits, and, along with other vulnerabilities, allows the encrypted traffic to be cracked within minutes on a standard laptop.

Along with this, researchers found other vulnerabilities with TETRA methods that released sensitive information — including within historical communications. The core vulnerability involved a jump-off from the main interface on the radio, and then which followed through with running malicious code execution on the process and then onto the signal processor and wifi hardware. This main chip on the device then contains a secure enclave, which stores the main encryption keys. The team were able to access this chip and discover the cryptography methods used and associated artefacts. For this, they have dubbed the vulnerability TETRA:BURST [here]:

The reduced security method of TEA1 was discovered as having an encryption key of just 80 bits (normally, we would use a 128-bit key size, at least). A key size of 80 bits puts it within a range which can be cracked using GPU clusters. But, the research team found a “secret reduction step” which supported lower levels of randomization for the encryption key and which significantly reduced the key strength. Using this, the team were able to crack the communication with consumer-level hardware and with inexpensive radio equipment. Ultimately, the researchers define the attack as fairly trivial to implement.

Vulnerabilities discovered

A number of CVEs have already been defined for the vulnerabilities. These are [here]:

• CVE-2022–24401. This involved the Air Interface Encryption (AIE) keystream generator allows for decryption oracle attacks.
• CVE-2022–24402. This relates to the backdoor of the 80-bit key on the TEA1 algorithm — and which allows a trivial cipher crack.
• CVE-2022–24404. This involves weaknesses in the AIE for malleability attacks.
• CVE-2022–24403. This is a weak cryptographic scheme that allows attackers to deanonymize and track users.
• CVE-2022–24400. This allows attackers to set the Derived Cypher Key (DCK) to 0.

On the CVE database [here], these vulnerabilities are marked as “** RESERVED **” and will be populated soon.

Conclusions

What we have here is “Security by obscurity” and not “Security by design”. It is difficult to keep anything a secret these days, and, as much as possible, methods should be open to assessment. Along with this, the reduction in the security level for TEA1 is causing major problems — just the Netscape restriction on TLS left us with a security legacy that took decades to address.