Inherent Vulnerabilities in Hybrid CDMA & Cryptographic Spread Spectrum for Space Systems

Authors: Edd Salkield, Sebastian Köhler, Simon Birnbach and Ivan Martinovic

Direct Sequence Spread Spectrum (DSSS) is widely used to improve satellite mission availability against unintentional interference and protect telecommand links against jamming, eavesdropping, and spoofing. Whilst current standards focus on cooperative Code Division Multiple Access (CDMA) DSSS methods, high-value government and military assets increasingly use cryptographic DSSS to improve security. Work is currently ongoing to bring cryptographic DSSS into the next revisions of the ETSI standards, but it has been found that cryptographic DSSS is significantly worse at multiple access than the currently standardized methods. In this context, the European Space Agency has studied a hybrid CDMA/cryptographic DSSS construction designed to simultaneously provide multiple-access and security. In this paper we perform the first systematic analysis of the hybrid protocol and discover a number of major design flaws which seriously degrade the security of the system. In particular, we find that reuse of the cryptographic spreading sequence leads to a catastrophic failure wherein all satellites’ data sequences can be recovered given knowledge of any single satellite’s data sequence. This also enables sufficient recovery of the spreading sequence to spoof arbitrary messages, and increases vulnerability to optimized jamming. We evaluate and validate these findings through simulations with respect to real-world systems, and use this to propose countermeasures and system improvements which should be considered as standardization work continues.