AUSF — Authentication Server Function¶

Learning objectives¶

After reading this page, you will be able to:

• Explain in one sentence the role the AUSF plays in the 5GC (the server side of UE authentication).
• Diagram how the AUSF cooperates with the AMF and UDM to carry out authentication.
• Explain at which point the AUSF appears within the Registration procedure.
• Explain that 4G (EPC) had no dedicated authentication server NF (it was split off as the AUSF).

Prerequisites¶

It will go more smoothly if you understand the following first:

• Basics of SBA (Service Based Architecture) and NFs → Curriculum
• The overall picture of authentication and the concept of keys such as KSEAF → Protocol dictionary
• The role of the AMF (the side that requests authentication from the AUSF) → AMF

What you will learn in this chapter¶

We enter from Why (what goes wrong without the AUSF), then organize What (its role, retained information, and APIs) with tables and Mermaid diagrams, and move to How (its appearance in Registration, comparison with EPC, and reference specs).

Why — why it is needed¶

Unless the network confirms "is this device really the genuine subscriber?", it cannot prevent impersonation and unauthorized connections. Something is needed that strictly performs this identity check (authentication) following a fixed procedure (such as 5G-AKA) — an authentication examiner. That is the AUSF.

Without the AUSF, the AMF (reception) cannot perform identity verification, and anyone could register. Conversely, it is precisely because the AUSF completes authentication and derives and hands over a shared key (KSEAF) that subsequent communication can be encrypted and protected.

Analogy: The AUSF is the building's security room (in charge of identity verification). Reception (the AMF) forwards a visitor's identification to the security room. The security room verifies its authenticity by referring to the HR department (UDM), which holds the employee roster, and replies to reception, "Identity confirmed. You may let them through with this key." The security room itself does not keep the original roster (that is HR's job).

Overview¶

The AUSF (Authentication Server Function) is the NF that handles the server side of UE authentication (5G-AKA / EAP-AKA'). It receives authentication requests from the AMF, retrieves and verifies authentication vectors in cooperation with the UDM, derives the key KSEAF, and returns it to the AMF. It holds no persistent subscriber data and handles only temporary information during the authentication session — an authentication-dedicated NF.

Basic Concept — explanation for beginners¶

The AUSF is like an exam proctor. When an examinee (the UE) claims "I am a legitimate examinee," the proctor (AUSF) receives the answer data (authentication vector) from headquarters (UDM) and checks the examinee's answers. If they pass, it issues proof that "this person is genuine" along with a passphrase (the key KSEAF) to be used afterward. The key point is that the proctor does not keep the original exam questions and fetches them from headquarters each time.

Note that the party receiving the KSEAF issued by the AUSF is the SEAF (Security Anchor Function). The SEAF is the function that serves as the security anchor, and it is co-located with the AMF. So the handoff is AUSF → (hands over KSEAF) → SEAF/AMF (basis: TS 33.501 §6).

Network Function details¶

Architecture¶

The AUSF stands between the AMF and the UDM, handling the relay and verification of authentication.

flowchart LR
  UE(("UE"))
  AMF["AMF"]
  AUSF["AUSF"]
  UDM["UDM"]

  UE -- "N1 (NAS)" --> AMF
  AMF -- "N12 (Nausf)" --> AUSF
  AUSF -- "N13 (Nudm)" --> UDM

How to read the diagram: The UE's authentication request reaches the AMF via NAS, and the AMF requests authentication from the AUSF over N12. The AUSF retrieves and verifies the authentication vector from the UDM over N13, and returns the result and key to the AMF. The AUSF never signals directly with the UE; it always goes through the AMF.

Interface¶

The main interfaces terminated by the AUSF and their roles.

Appearance in procedures¶

In Registration (initial registration), the AUSF appears at the stage where the AMF requests authentication. The AUSF retrieves an authentication vector from the UDM (N13) and verifies the UE's response following the 5G-AKA / EAP-AKA' procedure. On success, it derives KSEAF and returns it to the AMF (N12), laying the foundation for the subsequent establishment of NAS security.

See Registration for the detailed flow.

Key Hierarchy¶

5G security is built on a key hierarchy that, rooted in the per-subscriber permanent key K, derives child keys step by step during the authentication procedure. The AUSF holds KAUSF in the middle of this hierarchy and derives KSEAF to hand over to the SEAF/AMF — an important junction.

flowchart TB
  K["K (permanent key)
held by USIM / ARPF"]
  CKIK["CK / IK
derived at USIM / ARPF"]
  KAUSF["KAUSF
(held by AUSF)"]
  KSEAF["KSEAF
(derived by AUSF → to SEAF/AMF)"]
  KAMF["KAMF
(derived by SEAF/AMF)"]
  KNAS["KNASenc / KNASint
(NAS protection keys)"]
  KGNB["KgNB ...
(AS/radio-side keys)"]

  K --> CKIK
  CKIK --> KAUSF
  KAUSF --> KSEAF
  KSEAF --> KAMF
  KAMF --> KNAS
  KAMF --> KGNB

How to read the diagram (upper = root, lower = derived):

• K … the per-subscriber permanent key. Held by the USIM side and, on the network side, by the ARPF (which logically belongs to the UDM).
• CK / IK … the cipher key/integrity key derived in the AKA computation.
• KAUSF … the key held by the AUSF. This is the AUSF's area of responsibility.
• KSEAF … the anchor key derived by the AUSF and handed over to the SEAF (co-located with the AMF).
• KAMF … derived by the SEAF/AMF. The parent of subsequent NAS/AS keys.
• KNASenc / KNASint / KgNB … … the keys used for actual encryption, integrity protection, and radio-side protection.

Basis: 3GPP TS 33.501 §6.2 (Key hierarchy). The details of each key's derivation function (KDF) and input parameters are to be confirmed.

Comparison with EPC¶

• In 4G: Authentication was handled by the MME and HSS, and no dedicated authentication server NF existed. The MME retrieved authentication vectors (AV) from the HSS and exchanged AKA directly with the UE. Holding the permanent key and generating authentication vectors was handled by the AuC (Authentication Centre) within the HSS.
• In 5G: The authentication server function was made independent as the AUSF. By separating the AMF (access management), authentication (AUSF), and subscriber data (UDM/UDR), it achieves clarity of roles and servicification (SBA). The function corresponding to the 4G AuC is handled by the ARPF (which logically belongs to the UDM).

Release differences¶

• Rel-15: Defines the AUSF's basic functions (the server side of 5G-AKA / EAP-AKA', Nausf).
• Rel-16: Presence of functional extensions and section numbers are to be confirmed.
• Rel-17: Presence of functional extensions and section numbers are to be confirmed.

3GPP Specification¶

• 3GPP TS 23.501 §6.2.8 — AUSF functional definition (role of the Network Function)
• 3GPP TS 29.509 — Nausf service (UEAuthentication). Individual section numbers are to be confirmed
• 3GPP TS 33.501 — authentication procedures (5G-AKA / EAP-AKA', KSEAF derivation). Individual section numbers are to be confirmed

Summary¶

• The AUSF is the authentication-dedicated NF that handles the server side of UE authentication (5G-AKA / EAP-AKA').
• It receives authentication requests from the AMF, derives KSEAF in cooperation with the UDM, and returns it to the AMF.
• It holds no persistent subscriber data and handles only temporary authentication session information.
• Its terminating interfaces are N12 (AMF) and N13 (UDM).
• 4G had no dedicated authentication server NF; it was split off as the AUSF in 5G.

Next Step¶

• UDM — the party the AUSF retrieves authentication vectors from
• AMF — the party that requests authentication from the AUSF
• Registration — the procedure in which the AUSF appears during authentication
• NF dictionary / Cause dictionary — for checking terminology and failure causes