Cell Phone Location Privacy
We all know that our cell phones constantly give our location away to our mobile network operators; that’s how they work. A group of researchers has figured out a way to fix that. “Pretty Good Phone Privacy” (PGPP) protects both user identity and user location using the existing cellular networks. It protects users from fake cell phone towers (IMSI-catchers) and surveillance by cell providers.
It’s a clever system. The players are the user, a traditional mobile network operator (MNO) like AT&T or Verizon, and a new mobile virtual network operator (MVNO). MVNOs aren’t new. They’re intermediaries like Cricket and Boost.
Here’s how it works:
- One-time setup: The user’s phone gets a new SIM from the MVNO. All MVNO SIMs are identical.
- Monthly: The user pays their bill to the MVNO (credit card or otherwise) and the phone gets anonymous authentication (using Chaum blind signatures) tokens for each time slice (e.g., hour) in the coming month.
- Ongoing: When the phone talks to a tower (run by the MNO), it sends a token for the current time slice. This is relayed to a MVNO backend server, which checks the Chaum blind signature of the token. If it’s valid, the MVNO tells the MNO that the user is authenticated, and the user receives a temporary random ID and an IP address. (Again, this is now MVNOs like Boost already work.)
- On demand: The user uses the phone normally.
The MNO doesn’t have to modify its system in any way. The PGPP MVNO implementation is in software. The user’s traffic is sent to the MVNO gateway and then out onto the Internet, potentially even using a VPN.
All connectivity is data connectivity in cell networks today. The user can choose to be data-only (e.g., use Signal for voice), or use the MVNO or a third party for VoIP service that will look just like normal telephony.
The group prototyped and tested everything with real phones in the lab. Their approach adds essentially zero latency, and doesn’t introduce any new bottlenecks, so it doesn’t have performance/scalability problems like most anonymity networks. The service could handle tens of millions of users on a single server, because it only has to do infrequent authentication, though for resilience you’d probably run more.
The paper is here.