IMSI Pseudonymization

The IMSI Pseudonymization project will design a specification and provide a reference implementation of a mechanism to conceal the IMSI (international mobile subscriber identity) of a mobile subscriber on the radio interface. The IMSI is used to uniquely identify each subscriber in a (2G, 3G, 4G, 5G) cellular network. However, the privacy of users is not really well protected: current specification require to transfer the IMSI in plain-text at various times before an encrypted connection can be set up. The present project will specify, implement and evaluate a method by which the IMSI will be concealed on the air interface with no modifications to existing mobile phones or any network elements of the operator beyond the HLR/HSS (which implements the authentication on the network side).

Find the specification and SIM applet source code here. The related OsmoHLR branch is here.

This project is funded by NLnet and the NGI0 PET Fund.


  1. Create draft specification (completed, see imsi-pseudo-spec.pdf)
  2. Implement in Osmocom <-- we're here
  3. Update specification with experience from implementation
  4. Submit proposal for optional IMSI pseudonymization into the 3GPP standardization process


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