  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.
