The research focused on the GMR-2 algorithm
that is commonly used by modern-day satellite phones, including
Inmarsat, to encrypt voice calls in an attempt to prevent eavesdropping.
The attack method helped researchers effectively reduce the search space for the 64-bit encryption
key, which in turn made it easier to hunt for the decryption key,
resulting in the encrypted data to be cracked within a fraction of a
second.
The technique contains three phases, namely table generation; dynamic
table looks-up, filtration and combination; and verification. The attack
can be used to “retrieve the complete 8-byte encryption-key from only 1
frame (15 bytes) of keystream on average.” It also significantly
reduces the exhaustive search space, and requires only 6KB of extra storage space.
The security researchers reveal that, in 10,000 experiments, the newly devised technique was able to uniquely determine 97.2% of the encryption-keys by the 15 bytes of keystream. The remaining 2.8% of the keys needed an extra keystream byte to retrieve.
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