GMR-2 issues allow satellite phone communications decryption in near real-time

Researchers have exploited vulnerabilities in the implementation of the GMR-2 cipher decrypt satellite phone communications in fractions of a second.

Two Chinese security researchers have exploited vulnerabilities in the implementation of the GMR-2 standard that could be exploited to decrypt satellite phone communications in fractions of a second.

The GMR-2 is a stream cipher with 64-bit key-length being used in some Inmarsat satellite phones.

It is possible to crack the GMR-2 cipher by using only one frame known keystream, but the process is time-consuming. The researchers devised a technique that for the first time allows a real-time inversion attack using one frame keystream.

“Then by introducing a new concept called “valid key chain” according to the cipher’s key schedule, we
for the first time propose a real-time inversion attack using one frame keystream. This attack contains three phases: (1) table generation (2) dynamic table looks-up, filtration and combination (3) verification.” reads the research paper.

The satellite phones of British Telecom Inmarsat use the GMR-2 standard,

In 2012, a group of German researchers demonstrated through a reverse engineering activity that both are weaker than other ciphers such as AES or PRESENT.

“With respect to the GMR-2 cipher, in a known-plaintext setting where approximately 50–65 bytes plaintext are known to the attacker, it is possible to recover a session key with a moderate computational complexity, allowing the attack to be easily performed with a current PC,” explained the German experts.

The Chinese duo detailed a new real-time inversion attack against GMR-2 that allows attackers to obtain a real-time decryption of satellite communications.

Differently, from the previous attack method, the researchers don’t crack the cipher with a plaintext attack, instead, they reverse engineered the encryption process in order to extrapolate the encryption key directly from the output keystream.

“Our analysis shows that, using the proposed attack, the exhaustive search space for the 64-bit encryption key can be reduced to about 213 when one frame (15 bytes) keystream is available. Compared with previous known attacks, this inversion attack is much more efficient. Finally, the proposed attack are carried out on a 3.3GHz platform, and the experimental results demonstrate that the 64-bit encryption-key could be recovered in around 0.02s on average.” continues the paper.

The security of satellite communication is crucial and the choice strong algorithms is essential to avoid eavesdropping.

“Given that the confidentiality is a very crucial aspect in satellite communications, the encryption algorithms in the satellite phones should be strong enough to withstand various eavesdropping risks,” continues the analysis.

“Table 3 is the comparison between the known cryptanalytic results and ours, from which we can see
that the inversion attack proposed in this paper possesses evident superiority compared with the dynamic guess-and-determine attack and the read-collision based attack. Given one frame (15 bytes) of keystream, one can break the GMR-2 cipher with only 0.02s on a 3.3GHz platform. This again demonstrates that there exists serious security flaws in the GMR-2 cipher, and it is crucial for service providers to upgrade the cryptographic modules2 of the system in order to provide confidential communication.”

Of course, satellite phone providers must upgrade their systems in order to protect communications.

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Pierluigi Paganini 

(Security Affairs – satellite communications, hacking)

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