How it works5G-AKA in one paragraph
In 5G, primary authentication is mutual: the network proves it knows the subscriber's long-term key K, and the UE proves the same. The home network (UDM/ARPF) builds an authentication vector from K and a fresh RAND; the UE's USIM recomputes the expected response and checks the network's AUTN. Unlike 4G, the home network (AUSF) makes the final decision, and the SUPI is concealed as a SUCI so it is never sent in the clear.
UDM/ARPF: 5G HE AV = (RAND, AUTN, XRES*, KAUSF)
AUSF: HXRES* = hash(RAND, XRES*) · derive KSEAF from KAUSF
UE: verify AUTN → compute RES → RES* → KAUSF, KSEAF
SEAF: HRES* = hash(RAND, RES*) =? HXRES* AUSF: RES* =? XRES*
KSEAF → KAMF → NAS & AS keys
FAQFrequently asked questions
Who decides authentication success in 5G-AKA?
The home network does. The SEAF first checks HRES* against HXRES* for a quick local check, but final confirmation is made by the AUSF comparing RES* with XRES* — a key change from 4G EPS-AKA, where the visited MME decided.
What is in the 5G authentication vector?
The 5G HE AV from UDM/ARPF contains RAND, AUTN, XRES* and K_AUSF. The AUSF turns it into a 5G SE AV (RAND, AUTN, HXRES*) for the SEAF.
What is the difference between RES* and XRES*?
XRES* is the expected response computed by the home network; RES* is the response computed by the UE/USIM. Authentication succeeds when they match.
Where does K_SEAF come from?
The UE and AUSF both derive K_AUSF from CK/IK, then derive K_SEAF from K_AUSF bound to the serving-network name. K_SEAF is the anchor key, from which K_AMF and the NAS/AS keys are derived.