5G AKA — How the device and the network prove themselves, and what they derive
The device — identity, state, mobility
Where it sits
What it is
What it is. The mutual authentication procedure between a device and its home network, and the key hierarchy it produces. TS 33.501 §6.1.3.2 for 5G AKA; §6.1.3.1 for the EAP-AKA′ variant. "Mutual" is the word to hold on to: the network proves itself to the device as well.
The sequence, at the level this folder needs:
- The device sends a SUCI (or a 5G-GUTI) in its Registration Request.
- The AMF, acting as SEAF — the security anchor — asks the AUSF in the home network to authenticate.
- The AUSF asks the UDM, which de-conceals the SUCI (the SIDF) and generates an authentication vector from the long-term key.
- A challenge reaches the device; the USIM computes the response using the key that never leaves it, and checks the network's authentication token — this is the step that authenticates the network.
- The response returns to the AUSF, which confirms, and an anchor key is delivered to the AMF.
- The AMF derives everything below it: NAS keys, and the access-stratum keys the PDCP layer uses.
Mutual authentication is what kills the false base station. A device that only proved itself could be lured onto any transmitter claiming to be a network. Because step 4 requires the network to produce something only the home network could have computed, an attacker without the long-term key cannot complete it. Combined with SUCI — which denies the attacker the identity in the first place — this is the pair of changes that closed the IMSI-catcher attack.
The hierarchy is rooted where the signalling is. Authentication terminates at the AUSF, in the home network, but the anchor key lands at the AMF, because the AMF is where the device's NAS connection lives and where the RAN's keys must be derived from. In roaming, the home network authenticates and the visited network gets a key — never the long-term secret.
And the keys are separated by purpose. NAS ciphering and integrity are the AMF's (TS 23.501 §6.2.1: "Termination of NAS (N1), NAS ciphering and integrity protection"); the access-stratum keys protect the radio and are used by PDCP. Compromising one layer does not hand over the other.
Where it sits in a device's life. Authentication happens during registration, and the resulting security context is then reused — TS 23.501 §5.3.2.2.2 notes that parts of the context are kept even in RM-DEREGISTERED "e.g. to avoid running an authentication procedure during every Registration procedure".
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: