digitalcerts

A few weeks ago I wrote a blog post about a 3GPP SA3 proposal to leverage Digital Certificates in cellular networks. Despite some aspects in the proposal being far from ideal (as I discussed in the blog post), that proposal was and is very good news. It is the first time I have seen an actual proposal to leverage PKI to address inherent security issues in cellular networks on both LTE and 5G.

Today, a new revision of that proposal has been published, and I am now one of the co-authors (under my Softhandover affiliation). The new revision of 3GPP SA3 S3-202630 can be found here.

proposal

This is a proposal for Non-Public Networks (NPN), which are essentially “private” 5G deployments commonly used for critical applications such as first responders, tactical networks, utilities and industrial plants, etc. As such, this proposal is not aimed at commercial 5G deployments. However, the technology proposed could fairly easily be applied to commercial 5G networks by expanding the Certificate Authority network. For example, a TelCo operator could sign certificates for foreign operators with which it has trusted roaming agreements, such that devices roaming could verify the certificate chain that a roaming base station presents to the UE.

One of my main concerns with the previous revision was that it proposed to encrypt broadcast messages as opposed to sign them.

The newest revision proposes to actually sign broadcast messages along with a digest that includes a time stamp and, potentially, some form of geo-location in order to prevent for those messages to be replayed. Along with the message, each base station passes to the UE a certificate signed by the core network which attests the base station’s public key.

By leveraging digital certificates and a root of trust pre-loaded on the UEs (in the case of private 5G networks, the core network as Certificate Authority) both broadcast messages (SIBx) and signaling messages (RRC, NAS, etc) can present a certificate to the UE. Then, the UE can cryptographically verify that they originated at a real and trusted base station.

I am not too familiar yet on how 3GPP procedures work when it comes to such proposals, so I am not sure why the introduction of the document still alludes to encrypting broadcast messages, tough. But the body of the proposal is updated with the aforementioned improvements.

I have been advocating for years for leveraging PKI-like technology in cellular as a solution to the many vulnerabilities that leverage pre-authentication messages. Back in February, during my talk at ShmooCon, I made the case once more. I am glad to see that things are finally starting to move in that direction. And I am a coauthor!

If this proposal is approved, it will be part of the actual 5G standard. Exciting to know that I might be able to contribute to the global standard for cellular communications to improve its security. Hopefully we can bring such a proposal to commercial 5G deployments as well.