It’s worth noting too that because the USIM handles all this the baseband module, and therefore the mobile handset itself, doesn’t know any of the secret sauce used to negotiate with the network. This prevents anyone from setting up their own radio network from going all Lionel Ritche and saying “ Hello, is it me you’re looking for” and having all the UEs attach to the malicious network. The Baseband module provides the ingredients, but the USIM uses it’s secret recipe / ingredients combo, known only to the USIM and HSS, to perform the authentication.īecause the card challenges the network it means we’ve got mutual authentication of the network. When it comes time to authenticate to the network, and authenticate the network itself, the baseband module sends the provided challenge information from the network to the USIM which does the crypto magic to generate responses to the authentication challenges issued by the network, and the USIM issues it’s own challenges to the network. When you fire up your mobile handset the baseband module in it communicates with the USIM application on the card. One of the Java applets on the card / microcontroller will be the software stack for a SIM, used in GSM networks to authenticate the subscriber.įor UMTS and LTE networks the card would have a USIM software stack allowing it to act as a USIM, the evolved version of the SIM.īecause it’s just software a single card can run both a USIM and SIM software stack, and most do.Īs I’m building an LTE network we’ll just talk about the USIM side of things. So it’s a SIM card right?Īs a bit of background the ever shrinking card we all know as a SIM is a “Universal integrated circuit card” – a microcontroller with it’s own OS that generally has the ability to run Java applets. So let’s take a bit of a dive into USIMs. So because of this, you have to have full control over the SIM and the network. LTE doesn’t allow you to just use any SIM to authenticate to the network, but instead relies on mutual authentication of the UE and the network, so the Network knows it’s talking to the right UE and the UE knows it’s talking to the right network. I’ve been working on private LTE recently, and one of the first barriers you’ll hit will be authentication.
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