A real-time inversion attack on the GMR-2 cipher used in the satellite phones
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The GMR-2 cipher is a type of stream cipher currently being used in some inmarsat satellite phones. It has been proven that such a cipher can be cracked using only one single-frame (15 bytes) known keystream but with moderate executing time. In this paper, we present a new thorough security analysis of the GMR-2 cipher. We first study the inverse properties of the cipher’s components to reveal a bad one-way character of the cipher. By then introducing a new concept called “valid key chain” according to the cipher’s key schedule, we propose an unprecedented real-time inversion attack using a single-frame keystream. This attack comprises three phases: (1) table generation; (2) dynamic table look-up, filtration and combination; and (3) verification. Our analysis shows that, using the proposed attack, the size of the exhaustive search space for the 64-bit encryption key can be reduced to approximately 213 when a single-frame keystream is available. Compared with previous known attacks, this inversion attack is much more efficient. Finally, the proposed attack is carried out on a 3.3-GHz PC, and the experimental results thus obtained demonstrate that the 64-bit encryption-key could be recovered in approximately 0.02 s on average.
Keywordssatellite phone stream cipher GMR-2 cryptanalysis inversion attack
The authors wish to thank the anonymous reviewers for their valuable suggestions and comments, which greatly improve the presentation and quality of the current paper. This work in this paper was supported by National Nature Science Foundation of China (Grant Nos. 61402515, 61672530).
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