Abstract
Modern smartphone messaging apps now use end-to-end encryption to provide authenticity, integrity and confidentiality. Consequently, the preferred strategy for wiretapping such apps is to insert a ghost user by compromising the platform’s public key infrastructure. The use of warning messages alone is not a good defence against a ghost user attack since users change smartphones, and therefore keys, regularly, leading to a multitude of warning messages which are overwhelmingly false positives. Consequently, these false positives discourage users from viewing warning messages as evidence of a ghost user attack. To address this problem, we propose collecting evidence from a variety of sources, including direct communication between smartphones over local networks and CONIKS, to reduce the number of false positives and increase confidence in key validity. When there is enough confidence to suggest a ghost user attack has taken place, we can then supply the user with evidence to help them make a more informed decision.
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Notes
- 1.
Communication between client and key server occurs over TLS, with the key server usually authenticated with a certificate, so there is still some residual dependence on clocks in this case.
- 2.
To avoid key changes, Signal allows the user to save a backup of the secret key on the old device, and restore it on the new device. However, this process is poorly documented and difficult to perform correctly. It requires the use of third-party apps to transfer and set up the backup before installing Signal on the new device.
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Acknowledgements
This work was supported by the Boeing Company and the Engineering and Physical Sciences Research Council (EPSRC) [grant numbers EP/M020320/1 and EP/M508007/1].
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Vasile, D.A., Kleppmann, M., Thomas, D.R., Beresford, A.R. (2020). Ghost Trace on the Wire? Using Key Evidence for Informed Decisions. In: Anderson, J., Stajano, F., Christianson, B., Matyáš, V. (eds) Security Protocols XXVII. Security Protocols 2019. Lecture Notes in Computer Science(), vol 12287. Springer, Cham. https://doi.org/10.1007/978-3-030-57043-9_23
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