Authentication in mobile networks is executed leveraging a symmetric key system. For each mobile subscriber, there is a secret key that is known only by the mobile device and the network operator. Actually, it is not the device itself holding the key, but the SIM card. On the network side, in the case of LTE, the secret key is stored in the Home Subscriber Server (HSS).

Based on this pre-shared secret key, a mobile device and the network can mutually authenticate itself. Though, this is not necessarily the case. For some reason someone must have thought, when designing 2G-GSM, that having the end point authenticate the mobile network was not a requirement… too bad that not having mutual authentication opens the door to all types of rogue base station MitM attacks. Bad things also happen when this pre-shared “secret” key is sent from the SIM card manufacturer to the mobile operator in the clear in a bunch of DVDs and someone manages to steal them.

After years or security research in mobile networks, identifying, implementing and testing protocol exploits, I started thinking that perhaps it would be a good idea to transition the security architecture of a mobile networks towards a PKI-based system. This is why I really enjoy reading research papers with PKI proposals for mobile networks, which is a rather rare topic in the research community. Thanks to Google Scholar, a very interesting paper showed up in my radar: Chandrasekaran, Varun, and Lakshminarayanan Subramanian. “A Decentralized PKI In A Mobile Ecosystem.

PKI would increase the complexity of each cryptographic operation, but it is not like device and network authenticate each other constantly. Definitively, a lot of research would have to be done to validate whether it would be possible.

With a PKI-based authentication architecture in mobile networks, so many cool things could potentially be done. For example, it is very well understood that, regardless of mutual authentication and strong encryption, a mobile device engages in a substantial exchange of unprotected messages  with *any* LTE base station (malicious or not) that advertises itself with the right broadcast information (and this broadcast information is transmitted in the clear in the SIB broadcast messages). And this is the source of a series of protocol exploits and attacks. Perhaps, by means of PKI, broadcast messages could be “signed” by the operator in a way that mobile devices could verify their freshness (to avoid replay attacks) and verify that the base station is legitimate. This would allow mobile devices to verify the legitimacy of a base station before starting to engage in RACH procedures, RRC connection establishments, NAS attach exchanges, etc.

Anyhow, very interesting paper on cool things that could be done applying PKI to mobile networks. Worth reading it.

 

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