USIM — The one thing in the device the operator actually trusts
The device — identity, state, mobility
Where it sits
What it is
What it is. The application on the card that holds the long-term secret — the key the network shares with this subscription and nothing else. TS 33.501 §6.12.2 and Annex C describe what it does in 5G. It is the one part of a device the operator actually trusts.
What is on it that cannot be on the phone:
the long-term key K, which never leaves it · the SUPI · the Home Network Public Key used to compute a SUCI · the identifier of which protection scheme to use · sequence numbers for replay protection.
The key never leaves, and that is the entire point. Authentication works by the network sending a challenge and the card computing a response inside itself. The phone's operating system never sees K, so compromising the phone does not yield the subscription. Every security property in 5G AKA rests on that one boundary.
5G gave the card a new job. In LTE the card authenticated and little else. In 5G it also conceals the identity: computing the SUCI requires the home network's public key, and TS 33.501 §6.12.2 requires the UE to use "the raw public key... that was securely provisioned in control of the home network". Provisioning it on the card is what makes it unforgeable by the device.
And the fallback is spelled out. §6.12.2: "The ME shall support the null-scheme. If the home network has not provisioned the Home Network Public Key in USIM, the SUPI protection in initial registration procedure is not provided. In this case, the null-scheme shall be used." So an old card in a new phone gets no identity protection at all — the privacy improvement is a property of the card, not of the network or the handset.
ME and USIM are separate in the specification's language, and deliberately: the Mobile Equipment is the phone, identified by a PEI; the USIM is the subscription, identified by a SUPI. Which computation happens on which side is a security decision, made explicitly, clause by clause.
Read on
This concept is read as part of one argument in ref-ue-identity, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: