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This paper is in the area of pseudonym-based enhancements of user identity privacy in mobile networks. Khan and Mitchell (2017) have found that in recently published pseudonym-based schemes an attacker can desynchronize the pseudonyms' state in the user equipment and in its home network. In this paper, we first show that by exploiting this vulnerability a botnet of mobile devices can kick out of service a large portion of the users of a mobile network. We characterize this novel DDoS attack analytically and confirm our analysis using a simulation. Second, we explain how to modify the pseudonym-based schemes in order to mitigate the DDoS attack. The proposed solution is simpler than that in Khan and Mitchell (2017). We also discuss aspects of pseudonym usage in mobile network from charging and regulatory point of view.

3GPP Release 15, the first 5G standard, includes protection of user identity privacy against IMSI catchers. These protection mechanisms are based on public key encryption. Despite this protection, IMSI catching is still possible in LTE networks which opens the possibility of a downgrade attack on user identity privacy, where a fake LTE base station obtains the identity of a 5G user equipment. We propose (i) to use an existing pseudonym-based solution to protect user identity privacy of 5G user equipment against IMSI catchers in LTE and (ii) to include a mechanism for updating LTE pseudonyms in the public key encryption based 5G identity privacy procedure. The latter helps to recover from a loss of synchronization of LTE pseudonyms. Using this mechanism, pseudonyms in the user equipment and home network are automatically synchronized when the user equipment connects to 5G. Our mechanisms utilize existing LTE and 3GPP Release 15 messages and require modifications only in the user equipment and home network in order to provide identity privacy. Additionally, lawful interception requires minor patching in the serving network.

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