Don't Trust Satellite Phones: A Security Analysis of Two Satphone Standards

There is a rich body of work related to the security aspects of cellular mobile phones, in particular with respect to the GSM and UMTS systems. To the best of our knowledge, however, there has been no investigation of the security of satellite phones (abbr. sat phones). Even though a niche market compared to the G2 and G3 mobile systems, there are several 100,000 sat phone subscribers worldwide. Given the sensitive nature of some of their application domains (e.g., natural disaster areas or military campaigns), security plays a particularly important role for sat phones. In this paper, we analyze the encryption systems used in the two existing (and competing) sat phone standards, GMR-1 and GMR-2. The first main contribution is that we were able to completely reverse engineer the encryption algorithms employed. Both ciphers had not been publicly known previously. We describe the details of the recovery of the two algorithms from freely available DSP-firmware updates for sat phones, which included the development of a custom disassembler and tools to analyze the code, and extending prior work on binary analysis to efficiently identify cryptographic code. We note that these steps had to be repeated for both systems, because the available binaries were from two entirely different DSP processors. Perhaps somewhat surprisingly, we found that the GMR-1 cipher can be considered a proprietary variant of the GSM A5/2 algorithm, whereas the GMR-2 cipher is an entirely new design. The second main contribution lies in the cryptanalysis of the two proprietary stream ciphers. We were able to adopt known A5/2 cipher text-only attacks to the GMR-1 algorithm with an average case complexity of 2
³²
steps. With respect to the GMR-2 cipher, we developed a new attack which is powerful in a known-plaintext setting. In this situation, the encryption key for one session, i.e., one phone call, can be recovered with approximately 50-65 bytes of key stream and a moderate computational complexity. A major finding of our work is that the stream ciphers of the two existing satellite phone systems are considerably weaker than what is state-of-the-art in symmetric cryptography.